tests/migration: Convert x86 boot block compilation script into Makefile

tests/migration: Convert x86 boot block compilation script into Makefile

The x86 boot block header currently is generated with a shell script.
To better support other CPUs (e.g. aarch64), we convert the script
into Makefile. This allows us to 1) support cross-compilation easily,
and 2) avoid creating a script file for every architecture.

Note that, in the new design, the cross compiler prefix can be specified by
setting the CROSS_PREFIX in "make" command. Also to allow gcc pre-processor
to include the C-style file correctly, it also renames the
x86-a-b-bootblock.s file extension from .s to .S.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Wei Huang <wei@redhat.com>
Message-Id: <1536174934-26022-2-git-send-email-wei@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>