flash: allow extra tool flags (e.g. dpcmd --vcc 1800)

[llogen]

Is your feature request related to a problem? Please describe.

dpcmd has three distinct write operations and the module hardcodes one of them:

flag	dpcmd name	behaviour
-u	--auto	read, compare, erase only the differing sectors, program those
-z	--batch	blank-check, erase the entire chip, program the whole file from 0
-p	--prog	program without erasing

The module always uses -u. That is a sensible default, but -z is what you want for a known-good full image or a chip in an unknown state, and -p when the flash was erased separately. There is no way to pick, and the current choice is stated only in a code comment.

Describe the solution you'd like

with:
  tool: dpcmd
  writeMode: batch     # auto (default) | batch | prog

• Unset resolves to auto, so existing configs are unchanged.
• flashrom/flashprog always use -w and have no equivalent; setting writeMode for them should be a config error rather than silently ignored.
• Document the modes and the generated command line per tool in pkg/module/flash/README.md.

Describe alternatives you've considered

A generic args: [...] passthrough. Rejected: the modes are mutually exclusive operations, so -z passed that way would collide with the -u the module already emits.