dragonboard410c: Migrate to using CONFIG_EXTRA_ENV_TEXT

With the exception of distro_boot support, we can move all of the rest
of the environment changes to come from CONFIG_EXTRA_ENV_TEXT and in
turn remove CONFIG_ENV_REFLASH.

Cc: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>