tools: Make invocation of host compiler correct

tools: Make invocation of host compiler correct

HOSTCC should be used in any of the tools inside the tools/ directory
instead of CC. That way it is possible to override both values from the
command line when building the Trusted Firmware and the tools at the
same time. Also, use HOSTCCFLAGS instead of CFLAGS.

Also, instead of printing the strings CC and LD in the console during
the compilation of the tools, HOSTCC and HOSTLD have to be used for
clarity. This is how it is done in other projects like U-Boot or Linux.

Change-Id: Icd6f74c31eb74cdd1c353583399ab19e173e293e
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>