ramips: get MAC addr from the encrypted partition (WG4хх223)

ramips: get MAC addr from the encrypted partition (WG4хх223)

This commit resolves #10062. Adds decryption of the Arcadyan WG4xx223
configuration partition (board_data)to get base MAC address from it.
As a result, after this change the hack with saving MAC addressees to
u-boot-env before installation of OpenWrt is no longer necessary.

This is necessary for the following devices:
- Beeline Smartbox Flash (Arcadyan WG443223)
- MTS WG430223 (Arcadyan WG430223)

Example:
+----------------+-------------------+------------------------+
|                |    MTS WG430223   | Beeline Smartbox Flash |
+----------------+-------------------+------------------------+
| base mac (mtd) | A4:xx:xx:51:xx:F4 | 30:xx:xx:51:xx:06      |
| label          | A4:xx:xx:51:xx:F4 | 30:xx:xx:51:xx:09      |
| LAN            | A4:xx:xx:51:xx:F6 | 30:xx:xx:51:xx:09      |
| WAN            | A4:xx:xx:51:xx:F4 | 30:xx:xx:51:xx:06      |
| WLAN_2g        | A4:xx:xx:51:xx:F5 | 30:xx:xx:51:xx:07      |
| WLAN_5g        | A6:xx:xx:21:xx:F5 | 32:xx:xx:41:xx:07      |
+----------------+-------------------+------------------------+

Collected statistic shows that the 2-4th bits of the 7th byte of the
WLAN_5g MAC are the constant (see #10062 for more details):
- Beeline Smartbox Flash - 100
- MTS WG430223 - 010

Signed-off-by: Mikhail Zhilkin <csharper2005@gmail.com>