post/Makefile: fix dependency problem with parallel builds

Parallel builds (using "make -jN") would occasionally fail with error
messages like
	ppc_4xxFP-objdump: string.o: File format not recognized
or
	post/libpost.a(cpu.o): In function `cpu_post_test':
	/home/wd/git/u-boot/work/post/lib_ppc/cpu.c:130: undefined reference to `cpu_post_test_string'
or similar. We now make sure to run the 'postdeps" step before
attempting to build the specific POST libraries.

Signed-off-by: Wolfgang Denk <wd@denx.de>