NOR: add optional "flash erase_address" sector padding

NOR: add optional "flash erase_address" sector padding

Add a NOR flash mechanism where erase_address ranges can be padded
out to sector boundaries, triggering a diagnostic:

  > flash erase_address 0x0001f980 16
  address range 0x0001f980 .. 0x0001f98f is not sector-aligned
  Command handler execution failed
  in procedure 'flash' called at file "command.c", line 647
  called at file "command.c", line 361
  >

  > flash erase_address pad 0x0001f980 16
  Adding extra erase range, 0x0001f800 to 0x0001f97f
  Adding extra erase range, 0x0001f990 to 0x0001fbff
  erased address 0x0001f980 (length 16) in 0.095975s (0.163 kb/s)
  >

This addresses what would otherwise be something of a functional
regression.  An earlier version of the interface had a dangerous
problem:  it would silently erase data outside the range it was
told to erase.  Fixing that bug turned up some folk who relied on
that unsafe behavior.  (The classic problem with interface bugs!)
Now they can get that behavior again.  If they really need it,
just specify "pad".

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>