tests/migration: Support cross compilation in generating boot header file

tests/migration: Support cross compilation in generating boot header file

Recently a new configure option, CROSS_CC_GUEST, was added to
$(TARGET)-softmmu/config-target.mak to support TCG-related tests. This
patch tries to leverage this option to support cross compilation when the
migration boot block file is being re-generated:

 * The x86 related files are moved to a new sub-dir (named ./i386).
 * A new top-layer Makefile is created in tests/migration/ directory.
   This Makefile searches and parses CROSS_CC_GUEST to generate CROSS_PREFIX.
   The CROSS_PREFIX, if available, is then passed to migration/$ARCH/Makefile.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Wei Huang <wei@redhat.com>
Message-Id: <1536174934-26022-3-git-send-email-wei@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>