cmake.bbclass: Rework compiler program variables for allarch
CMake projects can specify the NONE project type. Projects that do this
do not use any C or C++ compiler, this currently works fine with caveat
that when changing the machine/arch the compiler is different causing
signature hash differences.
To avoid the signature hash differences clear the associated C/CXX
compiler variables. In order to achieve this with overrides, simplify
the existing construction of the values using a python function and
variable setting and remove the anonymous variable setup.
Signed-off-by: Nathan Rossi <nathan@nathanrossi.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
