loader: set options before including bsd.init.mk

bsd.init.mk ends up including defs.mk so the per-arch options must be
set before including defs.mk, or else the global defaults will be
used and the per-arch ones will be ignored.

Although better, note that the usage of MK_FDT before the inclusion of
bsd.init.mk is incorrect but doesn't lead to build errors. This
circular dependency must be broken in order for this to work
correctly.

Reviewed by: imp
Sponsored by: Citrix Systems R&D