ARM: rpi: Add CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG support

Add CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG support and enable it to set
'board_rev' and 'board_name' envs.
'board_rev' can be used in scripts to determine what board we are running on
and 'board_name' for pretty printing.

Signed-off-by: Guillaume GARDET <guillaume.gardet@free.fr>

Cc: Stephen Warren <swarren@wwwdotorg.org>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>