board: siemens: iot2050: Split the build for PG1 and PG2

board: siemens: iot2050: Split the build for PG1 and PG2

Due to different signature keys, the PG1 and the PG2 boards can no
longer use the same FSBL (tiboot3). This makes it impossible anyway to
maintaine a single flash.bin for both variants, so we can also split the
build.

A new target is added to indicates the build is for PG1 vs. PG2 boards.
Hence now the variants have separated defconfig files.

The runtime board_is_sr1() check does make no sense anymore, so remove
it and replace with build time check.

Documentation is updated accordingly. New binary artifacts are already
available via meta-iot2050.

Signed-off-by: Su Baocheng <baocheng.su@siemens.com>
[Jan: refactor config option into targets, tweak some wordings]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>