mach-snapdragon: implement capsule update support

mach-snapdragon: implement capsule update support

Qualcomm boards flash U-Boot a variety of partitions, implement support
for determining which slot U-Boot is running from, finding the correct
partition for that slot and configuring the appropriate DFU string.

Initially, we only support the RB3 Gen 2 where U-Boot is flashed to the
UEFI partition, and ignore handling of slots. In the future we will
additionally support booting U-Boot from other partitions (e.g. boot)
and correct handling for A/B.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>