bcm4908: build bootfs image per-SoC

bcm4908: build bootfs image per-SoC

In theory we could have just 1 bootfs image for all devices as each
device has its own entry in the "configurations" node. It doesn't work
well with default configuration though.

If something goes wrong U-Boot SPL can be interrupted (by pressing A) to
enter its minimalistic menu. It allows ignoring boardid. In such case
bootfs default configuration is used.

For above reason each SoC family (BCM4908, BCM4912) should have its own
bootfs built. It allows each of them to have working default
configuration.

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