toolchain-external: convert to the package infrastructure

This commit converts the 'toolchain-external' logic to the package
infrastructure.

The TOOLCHAIN_EXTERNAL_DIR variable (which points to where the
toolchain is located) is renamed to TOOLCHAIN_EXTERNAL_INSTALL_DIR,
because the former conflicts with the package infrastructure (which
defines the <pkg>_DIR variable for each package as pointing to its
build directory).

The new _EXTRA_DOWNLOADS mechanism is used for Blackfin toolchains.

The extract, configuration and installation steps are converted inside
the <pkg>_EXTRACT_CMDS, <pkg>_CONFIGURE_CMDS and
<pkg>_INSTALL_STAGING_CMDS.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>