cmake: in SDK use OE env var to set default toolchain
Patch the location in cmake where the toolchain file is loaded
to use the (new) OE_CMAKE_TOOLCHAIN_FILE variable to select a default
toolchain if no toolchain has been specified. The cmake alias is
removed.
The alternatives:
- shell alias fails when cmake is called indirectly (ex: a makefile
managing several projects which calls cmake for some of them)
because aliases are not inherited
- wrapper script that unconditionally adds "-D..." breaks cmake's
build tests and many other things as it causes cmake to believe it
should be configuring things when it should not be. For example,
`cmake -DCMAKE_TOOLCHAIN_FILE=... --build .` does not work (note
that this also breaks people directly using `cmake --build .` with
the current alias).
Signed-off-by: Cody P Schafer <dev@codyps.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
