Makefile: evaluate CCACHE and HOST{CC, CXX} at time of use

Makefile: evaluate CCACHE and HOST{CC, CXX} at time of use

As we are going to move to per-package SDK, the location of CCACHE and
therefore the definitions of HOSTCC and HOSTCXX need to be evaluated
at the time of use and not at the time of assignment. Indeed, the
value of HOST_DIR changes from one package to the other.

Therefore, we need to change from := to =.

In addition, while doing A := $(something) $(A) is possible, doing A =
$(something) $(A) is not legal. So, instead of defining HOSTCC in
terms of the current HOSTCC variable, we re-use HOSTCC_NOCCACHE
instead.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>