kbuild: improve check-symlink

o if include/asm point to a nonexisting directory remove the asm symlink
o if include/asm is a directory error out

This fixes a situation where one could be left with a symlink
to asm-x86 but that directory no longer exist and thus the build
would error out.

include/asm may be a directory if the kernel tree has been copied

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>