merge: support 'identical' on overlapped data

[MarcFinetRtone]

When merging multiple .hex files, in case of overlap we sometimes need to just 'ensure' that values are identical.

The new 'check' mode ensures this.

Note: the behavior for start_addr is currently a 'check' (i.e. the overlap configuration is checked only when values differ). So we might want to change the behavior for data too:

• if data is identical: silently bail out
• add a 'stricter' mode that corresponds to the current behavior

[MarcFinetRtone]

v2:

• replace 'check' with 'identical'
• fix test
• fix doc