arm: socfpga: DE1 use environment in common header

This removes the default environment from the de1 headers
and instead uses the common environment provided in
socfpga_common.h which now uses distro boot.

This board does not have a devicetree in the upstream kernel
source so set devicetree to socfpga_cyclone5_de1_soc.dtb.

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

--
Changes in V2:
 - Remove unneeded CONFIG_BOOTFILE
 - set devicetree name to match socfpga_{fpga model}_{board model}.dts
   pattern