ARM: rpi_*_defconfig: Add support to find UUID for filesystem

ARM: rpi_*_defconfig: Add support to find UUID for filesystem

The most generic way of having a stable boot behavior is to rely on
UUID instead of device names for root partition, so the order of
probing does not cause issues with booting.

This enables the `CMD_FS_UUID` for following defconfig files:

 - rpi_0_w_defconfig
 - rpi_2_defconfig
 - rpi_3_32b_defconfig
 - rpi_3_defconfig
 - rpi_defconfig

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Fabio Berton <fabio.berton@ossystems.com.br>
Signed-off-by: Alexander Graf <agraf@suse.de>