iot2050: Update firmware layout

iot2050: Update firmware layout

The latest version of the binary-only firmware parts come in a combined
form of FSBL and sysfw containers. This implies some layout changes to
the generated firmware image but also makes handling of artifacts much
simpler (4 files less). The env locations will not change, just the
space reserved for U-Boot will shrink from 4 to 3 MB - still plenty of
space left in practice.

Adjust configuration and documentation accordingly.

Along this change, add a new reservation for update commands of the
user-controlled OTP part. A specific userspace tool will fill it, and
the FSBL will evaluate it during boot. This reservation will use 64K of
the former sysfw section.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>