Ignore C library files when checking coding style

Ignore C library files when checking coding style

The C library source files embedded into the Trusted Firmware
tree are not required to comply to the Linux Coding Style.
Unfortunately, 'make checkpatch' does take them into account.

This patch modifies the Makefile so that the C library source
and header files are now ignored by 'make checkpatch'.

It also instructs the checkpatch.pl script to not treat the
presence of a 'Change-Id' line in the commit message as an error.

Change-Id: I38196202efe518bae3a57c2affe2ed7758c9f69c