stand: Fix kboot issue on powerpc64 with MK_LIB32=yes

When MK_LIB32 is true, we descend into stand for make includes, make
clean, etc. We shouldn't do this, so set MK_BOOT=no when we're building
includes.

Fixes: 6497250f6f7f
PR: 261497
Sponsored by: Netflix
Reviewed by: emaste, brooks (he suggested this)
Differential Revision: https://reviews.freebsd.org/D34141