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

Makefile.libcompat

@@ -27,7 +27,8 @@ LIBCOMPATWMAKEFLAGS+= CC="${XCC} ${LIBCOMPATCFLAGS}" \
 		-DNO_CPU_CFLAGS \
 		MK_CTF=no \
 		-DNO_LINT \
-		MK_TESTS=no
+		MK_TESTS=no \
+		MK_BOOT=no
 LIBCOMPATWMAKE+=	${TIME_ENV} ${LIBCOMPATWMAKEENV} ${MAKE} ${LIBCOMPATWMAKEFLAGS} \
 			OBJTOP=${LIBCOMPAT_OBJTOP} \
 			OBJROOT='$${OBJTOP}/' \