sunxi: add fdtoverlay_addr_r environment variable

Commit 69076dff ("cmd: pxe: add support for FDT overlays") added
support for loading DT overlay files to PXE boot. However, it needs
additional environment variable which points to memory location which
can be used to temporary store overlay data.

Add it and in the process unify alignment using spaces and fix comment.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>