mkimage: sunxi_egon: refactor for multi-architecture support

Refactor some functions in mkimage sunxi_egon type, in order to prepare
for adding support for more CPU architectures (e.g. RISC-V). In
addition, compatibility for operation w/o specified architecture is
kept, in this case the architecture is assumed as ARM.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>