scripts/dts: Make extract/flash.py easier to read

Clean up the code a bit:

 - Simplify the loops over the flash 'reg' properties by using range()

 - Build identifier names with a plain .format() where possible. This
   makes them stand out better in the code.

 - Remove redundant variables

 - Move variables close to where they're used

 - Misc. other minor improvements

generated_dts_board.conf and generated_dts_board_unfixed.h were verified
to be identical for disco_l475_iot1 and frdm_kw41z before and after the
cleanup.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>