fdtdec: allow board to provide fdt for CONFIG_OF_SEPARATE

fdtdec: allow board to provide fdt for CONFIG_OF_SEPARATE

Similar to CONFIG_OF_BOARD, but in this case the fdt is still built by
u-boot build.  This allows the board to patch the fdt, etc.

In the specific case of dragonboard 410c, we pass the u-boot generated
fdt to the previous stage of bootloader (by embedding it in the
u-boot.img that is loaded by lk/aboot), which patches the fdt and passes
it back to u-boot.

Signed-off-by: Rob Clark <robdclark@gmail.com>
[trini: Update board_fdt_blob_setup #if check]
Signed-off-by: Tom Rini <trini@konsulko.com>