xtensa: rename BUILTIN_DTB to BUILTIN_DTB_SOURCE

When building some xtensa config, I hit the following warning:
  drivers/staging/mt7621-dts/Kconfig:4:warning: 'BUILTIN_DTB' has wrong type.
  'select' only accept arguments of bool and tristate type

It is due to some arch use BUILTIN_DTB as a flag for the need to builtin
dtb but xtensa use it as a string for which dtb to bulltin.
But for this (which dtb to build), it is better to use
BUILTIN_DTB_SOURCE like other arch do.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>