toolchain-external: move parts to pkg-toolchain-external.mk

toolchain-external: move parts to pkg-toolchain-external.mk

pkg-toolchain-external.mk will be used later to define the
toolchain-external-package infra. Most of the variable and macro
definitions are shared with the legacy generic-package based
toolchain-external. Move these to pkg-toolchain-external.mk.

pkg-toolchain-external.mk is included implicitly by the include
toolchain/*/*.mk in the top-level Makefile. The order of inclusion is
not defined, but that doesn't matter because none of the variables
defined in pkg-toolchain-external.mk are used in conditions or in
rules in toolchain-external.mk, only in recursively-expanded
variables.

No functional changes at all. The output of 'make -qp' hasn't changed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Romain Naour <romain.naour@gmail.com>
[Arnout: split off into separate patch]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>