imx-bootlets: create files in the sstate input area

do_deploy_append was creating files in the sstate output directory
(DEPLOY_DIR_IMAGE).  Thus, files created there won't actually go to
the sstate data.

This issue leads to a subtle bug: if imx-bootlets had been built, so
it was in the sstate cache, do_rootfs would fail after TMPDIR was
removed, since it would run IMAGE_CMD_linux.hab.sb (from
image_types_reach.bbclass), which expects the files generated by
imx-bootlets.bbappend to be in TMPDIR/deply/image/<machine>.   The
files would not be there because do_deploy_append would create them in
DEPLOY_DIR_IMAGE, which is an sstate _output_ directory.  Once TMPDIR
is removed, files in TMPDIR/deply/image/<machine> would be gone, and
those files created by do_deploy_append won't be placed there again
because they were not written to the sstate input area.

The fix consists in creating files in an sstate _input_ directory
(DEPLOYDIR), as configured by deploy.bbclass, inherited by the
original imx-bootlets recipe.

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>