import: Link libnx for graphics applications in a kernel build.

[Fishwaldo]

Summary

• In a kernel build, every application built on NX or NXFONTS fails to
  link. The nx examples and fbcon alike come out with undefined references
  to the font and geometry routines.
• The link line for applications in a kernel build is -lmm -lc -lproxies.
  libnx is never named, even though the export has been shipping
  libnx.a all along, so nothing ever resolves against it.
• import/Make.defs now names it, ahead of -lc since libnx calls into
  libc, and only when CONFIG_NX is set so that configurations without
  graphics are unaffected.
• This is the build-system half of getting a graphics application running
  in a kernel build. fbcon needs one further, unrelated fix, which is a
  separate PR against this repo; I will link it here once it is open.
• No related issue filed.

Impact

• Is new feature added? Is existing feature changed? NO. Build fix.
• Impact on user? YES, positive. NX and NXFONTS applications become
  linkable in kernel builds. Nothing to adapt to.
• Impact on build? YES, that is the change: one more library on the
  application link line, and only when CONFIG_NX=y. Configurations
  without graphics see a byte-identical link line.
• Impact on hardware? NO.
• Impact on documentation? NO.
• Impact on security? NO.
• Impact on compatibility? NO. Additive; no existing link succeeds
  differently.
• Build-mode dependence? YES, CONFIG_BUILD_KERNEL only. The block
  edited is already inside ifeq ($(CONFIG_BUILD_KERNEL),y). Flat and
  protected builds do not use this link line and are untouched.

Testing

I confirm that changes are verified on local setup and works as intended:

• Build Host: macOS 26.5.1, arm64 (Apple Silicon), xPack riscv-none-elf-gcc
  15.2.0
• Target: RISC-V, ESWIN EIC7700X EVB (downstream board port, not yet
  upstream), kernel build, CONFIG_NX=y, CONFIG_NXFONTS=y

Testing logs before change, linking examples/fbcon:

/Users/fish/github/apps/examples/fbcon/fbcon_main.c:1142:(.text.fbcon_getglyph+0x20): undefined reference to `nxf_getbitmap'
/Users/fish/github/apps/examples/fbcon/fbcon_main.c:1062:(.text.fbcon_getglyph+0x164): undefined reference to `nxf_convert_32bpp'
/Users/fish/github/apps/examples/fbcon/fbcon_main.c:1536:(.text.fbcon_fillchar+0x46): undefined reference to `nxgl_rectintersect'
riscv-none-elf-ld: /Users/fish/github/apps/examples/fbcon/fbcon_main.c:1547:(.text.fbcon_fillchar+0x50): undefined reference to `nxgl_nullrect'

Every one of those symbols lives in libnx.a, which the export had already
installed.

Testing logs after change: the nxf_* and nxgl_* references resolve, the
application links, and it runs on the target:

[CPU0] display: /dev/fb0 1080p60
##### CMD 1: fbcon &
fbcon [0:100]
##### END 1 (ok, 1.57s)

Worth being precise about what this change does and does not fix: with only
this patch applied, fbcon's link gets past libnx and then fails on
g_builtin_count and g_builtins instead. That is a separate defect in
fbcon itself, fixed by the companion PR mentioned above. The run shown here
has both applied. Any other NX application, which does not consult the
builtin registry, links and runs with this patch alone.

PR verification Self-Check

• This PR introduces only one functional change.
• I have updated all required description fields above.
• My PR adheres to Contributing Guidelines and Documentation.
• My PR is still work in progress (not ready for review).
• My PR is ready for review and can be safely merged into a codebase.

---

Claude (claude-opus-5) assisted with diagnosing this and with authoring the
code comment and this PR description. The commit carries an Assisted-by: tag
per CONTRIBUTING.md §1.5.

[Fishwaldo]

The companion fbcon fix referred to above is now open as
#3708.

To restate the division, since the two were originally one commit: this PR is
the build-system fix and applies to every NX or NXFONTS application in a
kernel build. #3708 is a defect in examples/fbcon itself, which consults the
builtin registry that a kernel build does not have. They are independent and
can be reviewed and merged in either order; fbcon in particular needs both
before it links.