Make it explicit that a modifiable file is stored withing a
single 4K sector (for erase) and that this includes the 20
byte header.
When modifying a file pad up to the next section instead of
just to an alignment size. This enables future changes to be
more flexible in terms of alignment and padding. Although,
files/sections with different padding requirements will likely
get a different magic.
If the size of bootconf.txt is reduced then set the unused data
to all ones instead of leaving garbage at the end. The EEPROM
header contains the actual file but this makes it easier to
verify the image and makes overreads more obvious.
