Make scripts and Makefiles POSIX compliant

The bash builtin versions of the "test" (resp. "[") command allow
using "==" for string comparisons, but POSIX compatible implemen-
tations (like /usr/bin/test) insist on using "=" only. On such systems
you will see:

	$ /usr/bin/test a == a && echo OK
	/usr/bin/test: ==: binary operator expected

This patch fixes Makefiles and scripts to use POSIX style.

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