boards: Consolidate all TFM signing code

Place the new signing code in the TFM module CMakeLists.txt.
Make some small tweaks and add a sentence to the docs.

In the process, make a few changes to the signing code:
- Change some names of files created.
- Minimize the number of files created.
- Use hex files instead of bin files. This is so we don't need to know
  the offset when creating hex files from bin files.

Also add signing for MCUBOOT_IMAGE_NUMBER=1 based on the code from the
v2m_musca_b1 board, though, this board does not build with =1 now
because of (I assume) some flash aliasing which places the S and NS
images 0x10000000 apart, where the manual algorithm places them next to
each other. It builds with =2, though.

Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>