core: allow having a list of "cmake" candidates

core: allow having a list of "cmake" candidates

Add the BR2_CMAKE_CANDIDATES variable, containing a list of candidates
to check and use as BR2_CMAKE, if possible.

This allows using "cmake3" on CentOS 7, whose default cmake corresponds
to version 2.8.12. Example:

    $ make BR2_CMAKE_CANDIDATES="cmake cmake3"

Signed-off-by: Carlos Santos <casantos@datacom.ind.br>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>