arm: socfpga: DE0 use environment in common header

This removes the default environment from the de0 headers
and instead uses the common environment provided in
socfpga_common.h which now uses distro boot.

In addition to the above, add support to boot from the custom
a2 type partition

Signed-off-by: Dalon Westergreen <dwesterg@gmail.com>
Acked-by: Marek Vasut <marex@denx.de>

--
Changes in v2:
 - Remove unneeded CONFIG_BOOTFILE