board: turris: Initialize serial# env

Store serial number from atsha cryptochip into the serial# env variable.
U-Boot automatically puts content of this variable into the root device
tree property serial-number when booting Linux kernel. Refactor turris
atsha code and from turris_atsha_otp_get_serial_number() function returns
directly string suitable for printing or storing into device tree. Because
during different boot stages is env storage read-only, it is not possible
to always store serial number into env storage. So introduce a new function
turris_atsha_otp_init_serial_number() which is called at later stage and
which ensures that serial number is correctly stored into env.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>