random: mark bootloader randomness code as __init

add_bootloader_randomness() and the variables it touches are only used
during __init and not after, so mark these as __init. At the same time,
unexport this, since it's only called by other __init code that's
built-in.

Cc: stable@vger.kernel.org
Fixes: 428826f5 ("fdt: add support for rng-seed")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>