at91: Update MEESC board support

This patch implements several updates:
-disable CONFIG_ENV_OVERWRITE
-add new hardware style variants and set the arch numbers appropriate
-pass the serial# and hardware revision to the kernel
-removed unused macros from include/configs/meesc.h
-fixed multiline comment style

Signed-off-by: Daniel Gorsulowski <Daniel.Gorsulowski@esd.eu>