unix/Makefile: Move include of mpconfigport.mk to before mkenv.mk.

So that if MICROPY_FORCE_32BIT is set mpconfigport.mk it influences
mkenv.mk.

unix/Makefile

-include ../py/mkenv.mk
 -include mpconfigport.mk
+include ../py/mkenv.mk
 
 # define main target
 PROG = micropython