By default the devicetree class adds padding to the generated .dtb
files, which can be needed by the bootloader. However it also pads
.dtbo files, which is not useful.
Don't apply padding to the overlay devicetrees.
To achieve this:
* move "-p ${DT_PADDING_SIZE}" to a new variable, DTB_BFLAGS (B for
"base")
* add "-p 0" to DTC_OFLAGS to disable padding for overlays
