Drop the imx-m7-demos dependency and build only the A55 boot target,
since Cortex-M firmware is board-specific and should not be selected as
a default SoC artifact. This follows the same cleanup done upstream in
meta-freescale commit 598f047ef112a0c88f699714cd10dc87c8460c5a.
https://onedigi.atlassian.net/browse/DEL-9882
Signed-off-by: Javier Viguera <javier.viguera@digi.com>
(cherry picked from commit 81cf596834382f87701d701277e13c97316de978)
d96004a043