The user can provide additional CFLAGS to use when building TF.
However, these custom CFLAGS are currently prepended to the
standard CFLAGS that are hardcoded in the TF build system. This
is an issue because when providing conflicting compiler flags
(e.g. different optimisations levels like -O1 and -O0), the last
one on the command line usually takes precedence. This means that
the user flags get overriden.

To address this problem, this patch separates the TF CFLAGS from
the user CFLAGS. The former are now stored in the TF_CFLAGS make
variable, whereas the CFLAGS make variable is untouched and reserved
for the user. The order of the 2 sets of flags is enforced when
invoking the compiler.

Fixes ARM-Software/tf-issues#350

Change-Id: Ib189f44555b885f1dffbec6015092f381600e560