Introduce `tf_snprintf`

This is a reduced version of `snprintf` that only supports formats '%d',
'%i' and '%u'. It can be used when the full `snprintf` is not needed in
order to save memory. If it finds an unknown format specifier, it
prints an error message and panics.

Change-Id: I2cb06fcdf74cda2c43caf73ae0762a91499fc04e
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
diff --git a/Makefile b/Makefile
index ddf8756..9c0ccc0 100644
--- a/Makefile
+++ b/Makefile
@@ -150,6 +150,7 @@
 
 BL_COMMON_SOURCES	+=	common/bl_common.c			\
 				common/tf_printf.c			\
+				common/tf_snprintf.c			\
 				common/${ARCH}/debug.S			\
 				lib/${ARCH}/cache_helpers.S		\
 				lib/${ARCH}/misc_helpers.S		\