Implement a leaner printf for Trusted Firmware

This patch implements a "tf_printf" which supports only the commonly
used format specifiers in Trusted Firmware, which uses a lot less
stack space than the stdlib printf function.

Fixes ARM-software/tf-issues#116

Change-Id: I7dfa1944f4c1e634b3e2d571f49afe02d109a351
Merge pull request #165 from soby-mathew/sm/lean_printf

Implement a leaner printf for Trusted Firmware