MIPS: Make the kernel arguments from dtb available

Similar to how arm allows using selecting between bootloader arguments,
dtb arguments and both, allow to select them on mips. But since we have
less control over the place of the dtb do not modify it but instead use
the boot_command_line for merging them.

The default is "use bootloader arguments" to keep the current behaviour
as default.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: Kevin Cernekee <cernekee@gmail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: John Crispin <blogic@openwrt.org>
Cc: Ganesan Ramalingam <ganesanr@broadcom.com>
Cc: Jayachandran C <jchandra@broadcom.com>
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: James Hartley <james.hartley@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/11284/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>