axs10x: Move environment from I2C EEPROM to SD-card

With deprecation of I2C EEPROM we we left without a permamnent
storage for U-Boot environment, but luckily we may simply use SD-card
with FAT partition for that.

Having environment on SD-card is much more convenient as it
allows us to preserve all the settings when moving from one board to
another. Moreover instead of 256 bytes of EEPROM we're now virtually
unlimited in stuff being placed in environment like complicated scripts
etc which are usually required in case of full-scale distros.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>