airoha: Get rid of unintended prints during sysupgrade

During image verification before sysupgrade, the line "bootloader-emmc"
was printed to the output of platform_check_image. When supplied with a
pre-7.1.0ALPHA3 image squashfs could be printed.  Silence the output of
the offending command.

Signed-off-by: Andreas Gnau <andreas.gnau@iopsys.eu>