toolchain-external: introduce toolchain-external-package

toolchain-external: introduce toolchain-external-package

The toolchain-external-package infrastructure is just a copy of the
toolchain-external commands, replacing TOOLCHAIN_EXTERNAL by $(2)
and adding double-dollars everywhere.

toolchain-external itself is converted to a virtual package, but it
is faked a little to make sue the toolchains that haven't been
converted to toolchain-external-package yet keep on working.

The TOOLCHAIN_EXTERNAL_MOVE commands don't have to be redefined
for every toolchain-external-package instance, so that is moved
out into the common part of pkg-toolchain-external.mk.

The musl-compat-headers dependency stays in the toolchain-external
package itself.

The musl ld link is duplicated in the legacy toolchain-external and
the toolchain-external-package, because they have separate hooks.

The handling of TOOLCHAIN_EXTERNAL_BIN deserves some special attention,
because its value will be different for different
toolchain-external-package instances. However, the value only depends
on variables that are set by Kconfig (BR2_TOOLCHAIN_EXTERNAL_PREFIX
and BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD) so it can easily be used in
the generic part. So we don't have to do anything specific for this
variable after all.

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