bcm4908: start work on images for devices using U-Boot

bcm4908: start work on images for devices using U-Boot

New BCM4908 family based routers will use U-Boot bootloader. That will
require using a totally different firmware format. Kernel has to be put
in a FIT image.

OpenWrt has some helpers for generating .its files but they don't fit
BCM4908 requirements and there is no simple way of extending any of
them. The best solution seems to be storing an .its template.

BCM4908 bootfs may:
1. contain extra binaries (other than kernel & DTB)
2. include multiple DTB files
3. store device specific U-Boot configurations with custom properties

Such setups are too complex to generate using shell script. Raw .its
file on the other hand seems quire clean & reasonable.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit a02c971dff934a11ddadf7ffd850dabcab5a3cdf)