mkexport: Allow boards to supply custom gnu-elf.ld.

[g2gps]

Summary

Allows boards to supply their own gnu-elf linker script, used when building Nuttx applications and CONFIG_BUILD_KERNEL.

This is useful when building fully-linked applications (see #9395).

Impact

Makes it easier to customize Nuttx userspace application link configuration.

Testing

Using the arty:knsh configuration:

$ make export -j 16
$ cd ../apps && ./tools/mkimport.sh -z -x ../nuttx/nuttx-export-*.tar.gz
$ diff -q import/scripts/gnu-elf.ld ../nuttx/binfmt/libelf/gnu-elf.ld

After placing a custom linker script at boards/risc-v/litex/arty_ay/scripts/gnu-elf.ld

$ make export -j 16
$ cd ../apps && ./tools/mkimport.sh -z -x ../nuttx/nuttx-export-*.tar.gz
$ diff -q import/scripts/gnu-elf.ld ../nuttx/binfmt/libelf/gnu-elf.ld
Files import/scripts/gnu-elf.ld and ../nuttx/binfmt/libelf/gnu-elf.ld differ