stand: Add MK_PIE=no to defs.mk

There's no need to build both pie and non-pie .o's for stand. There's
some other build thing with MK_BEAR_SSL=yes and/or MK_LOADER_VERIEXEC=yes
that causes the pie build to fail that the 'ar' stage now. Since we don't
need the PIE stuff and the non-PIE stuff, disable PIE for the boot loader.

Reviewed by: emaste
Sponsored by: Netflix