When MK_LIB32 is true, we descend into stand for make includes, make
clean, etc. However, we have a weird situation: we have the MK_*
settings for powerpc64 (so MK_LOADER_KBOOT=yes), but we have
MACHINE_ARCH=powerpc for the 32-bit build. Since we don't have a
kboot/arch/powerpc/Makefile.inc, that leads to an error when it can't be
found. Add a comment about this case and don't descend into kboot when
there's no Makefile.inc. I don't know of a better way to detect LIB32
build (in general, stand should be skipped for LIB32 builds, but that's
the same issue as here: how do we exclude it).
Fixes: 6497250f6f7f
PR: 261497
Sponsored by: Netflix