Feature: add log message for flash_prepare()

[ALTracer]

Detailed description

• This can be called a minor new feature.
• The existing problem is flash_prepare() functions of target drivers called transparently but, depending on specific flash controller, double unlocking (for erase, for write) may trigger potentially hard to detect errors.
• This PR adds a log line highlighting what function with what argument is invoked when in the flashing process.

I've used a similar patch to confirm speed of stlinkv3+STM32H743ZI reflashing operation with help of tio timestamping on ttyBmpTarg.
There could be more such logging extracted from per-target functions into central target_flash.c API layer, but a broader refactoring requires more effort. target_flash_erase() is logged, target_flash_write() isn't. I don't like how less readable this becomes, but this is the best place to log -- when all the conditions are passed but before the actual call (which may fail or hang or throw in future drivers). I could move the string array out of two branch scopes.
C has no enum->string reflection. Not urgent for v2.0.

Your checklist for this pull request

• I've read the Code of Conduct
• I've read the guidelines for contributing to this repository
• It builds for hardware native (see Building the firmware)
• It builds as BMDA (see Building the BMDA)
• I've tested it to the best of my ability
• My commit messages provide a useful short description of what the commits do

Closing issues

[dragonmux]

This LGTM, and we'll merge this once it's been rebased on main. There is one item we spotted, but we don't think it's a blocker - just a nice to have item to dot I's and cross T's.

Thank you for the contribution and helping improve the debugability of the Target Flash API!