libc/snprintf: add support to print "%" character

Enable snprintf()/vsnprintf() in TF-A to print "%" character as C
standard, which may be used in platform porting to print percentage
information.

Signed-off-by: Heyi Guo <guoheyi@linux.alibaba.com>
Change-Id: I9b296372a1002046eabac1df5e8eb99a27efd4a8
1 file changed