Fix the handling of MMU type in the AP entry code.  The ARM_MMU_V6/V7
symbols are always #defined to 0 or 1, so use #if SYM not #if defined(SYM).
Also, it helps if you include the header file that defines the symbols.
Allow the make_dtb script to work outside of a "make buildkernel" context
by setting MACHINE from uname -m if it's not set already.

Reviewed by:	imp, tuexen
Merge remote-tracking branch 'freebsd/master' into hardened/current/master