brcm47xx: extend firmware validation

brcm47xx: extend firmware validation

This provides TRX validation result, so final JSON may look like:
{
	"tests": {
		"fwtool_signature": true,
		"fwtool_device_match": true,
		"trx_valid": true
	},
	"valid": true,
	"forceable": true
}

It also prevents users from installing broken firmware files, e.g.:

root@OpenWrt:/# sysupgrade -F -n /tmp/TZ
Image metadata not found
Invalid image type. Please use firmware specific for this device.
Image check failed but --force given - will update anyway!
Commencing upgrade. Closing all shell sessions.
Firmware image is broken and cannot be installed

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>