boards: siemens: iot2050: Unify PG1 and PG2/M.2 configurations again

This avoids having to maintain to defconfigs that are 99% equivalent.
The approach is to use binman to generate two flash images,
flash-pg1.bin and flash-pg2.bin. With the help of a template dtsi, we
can avoid duplicating the common binman image definitions.

Suggested-by: Andrew Davis <afd@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>