toolchain/external: use generic extract commands (!blackfin case)

Now that packages can provide a list of files to be excluded when
extracting their archive, downloaded external toolchains are no longer
special in this respect.

Still, those toolchains are currently extracted directly into their
final location, $(HOST_DIR)/opt/ext-toolchain/ which means we still
need a custom extract command.

Except, we don't really need it: we can just move the toolchain, after
it's been extracted by the generic extract command, with a post-extract
hook.

This means that:

  - we now extract the toolchain with the generic extract command,

  - the toolchain is thus extracted into $(@D) ,

  - fixup commands are run against $(@D), as a post-extract hook,
    instead of against $(HOST_DIR)/opt/ext-toolchain ,

  - once this is done, we move $(@D)/* into the final location with a
    new post-extract hook.

Note: the blackfin case is special, and will be handled in a follow-up
patch.

[Thomas: register the TOOLCHAIN_EXTERNAL_FIXUP_CMDS only for the Arago
case, add some additional comments in the code about why we're moving
the toolchain around.]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Reviewed-by: Romain Naour <romain.naour@openwide.fr>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>