kernel-yocto.bbclass: do_kernel_metadata depends on do_unpack

kernel-yocto.bbclass: do_kernel_metadata depends on do_unpack

Make sure that 'do_unpack' is executed before 'do_kernel_metadata'.
Enabling externalsrc for kernel disables 'do_validate_branches' task
which caused 'do_kernel_metadata' to fail as the dependency chain to
'do_unpack' got broken.

[YOCTO #6658]

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>