font8x8: allow 1K font

We don't always want to blow 2K on fonts, so allow a 1K font option
px4plus: note the state of play
start: move more discardables into _DISCARD
binman: Teach binman some new tricks

Allow for discard being low
Allow for initializer being low (indicating we are not doing common packing
and the classic 'copy up' image)
Spot data/const overlapping for cases we force data start (PX4Plus)
Smart end/start identification for complex images rather than the classic
image.

Hopefully classic images still work
vt: We are going to make this more bankable, add warnings
px4plus: Add initial (untested) video, and adjust memory maps etc

Add an init.c for code we can discard.
crt0: Prepare for _VIDEO area
crt0.s: Prepare for _VIDEO
Makefile: introduce the _VIDEO segment, rebalance CODE and CODE2

Not that the linker can bank the things yet...