uboot-sign: Add support for setting firmware property in FIT configuration

uboot-sign: Add support for setting firmware property in FIT configuration

Add the ability to set the "firmware" property in the FIT configuration node
by introducing the UBOOT_FIT_CONF_FIRMWARE variable.

This property defines the primary image to be executed during boot. If it is
set, its value will be written into the FIT configuration under the "firmware"
field. If not set, the bootloader will fall back to using the first entry in
the "loadables" list.

Using this property improves control over the boot sequence, especially in
multi-binary boot scenarios.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>