libpayload: minor cleanups

Apply some const correctness to const/non-const strings in libc and
libpci (what an ugly cast that was).

Remove duplicated NULL test in printf_putstr(), already done in
print_string() - reduces size of libpayload by a few bytes.

Change-Id: I13f479df13e39d79cab291e9d99d153e1ef43eae
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Reviewed-on: http://review.coreboot.org/849
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marcj303@gmail.com>