Addendum to PR31124

Addendum to PR31124

This is a small addendum to PR31124 "rodata in flash for
more AVR devices".

It adds some symbols so the startup code can set a lock
on the FLMAP bit field as specified by the user.

New symbols are introduced because otherwise, all the
computations / decisions would have to be performed at
run-time.

It approved, please apply to master.

Johann

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ld/
	PR 31124
	* scripttempl/avr.sc: Adjust comments.
	[MAYBE_FLMAP]: Add symbols __flmap_value and __flmap_value_with_lock.
	Remove __flmap_lsl4.