cmake: move creation of toolchain file to package/pkg-cmaketargets.mk

This also requires moving a few include directives below the "all:"
target in the main Makefile, otherwise the new target to create the
toolchain file in pkg-cmaketargets.mk gets used as the default make
target instead of "all:".

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>