Implemented colored hex digits for machine code monitor

* main.asm: some test code to show the digits
* video.asm: added more symbols for external use
* hexdigits.asm: this is the new code ;-)
Some premature optimization ;-)

Moved the call to video_spi_wait before the writes.
Otherwise SPI would not operate asynchronously at all.
Minor syntax changes.

OR A, C is more readable than just OR C
Monitor needs the latest changes from the video branch: changed order of spi wait and write

Merge branch 'video' into monitor
added .video_spi_write_A shortcut to directly send the value in A to the video device
Merge branch 'video' into monitor

monitor needs the latest video interface (again) for directly writing values from A to the video device
machine code monitor now shows a hexdump :)

* Makefile: added new files
* hexdigits.asm: XREFs before XDEFs for consistency
* linechars.asm: new file, implements solid square characters
* main.asm: removed test code, jump into monitor
* monitor.asm: new file, implemented hex listing
* video.asm: some XDEFs, changed default background color
minor change: fixed case for constant
added single byte write routines, renamed spi_write to spi_transmit
Merge branches 'keyboard' and 'video'
the keyboard driver is now usable. some updates from video are needed for a simple test program.
small test program: print typed characters on screen.
removed some duplicate code (whoops...)
removed leftover comment
cleaned up code, repaired single byte video write
Merge branch 'master' into monitor

monitor now needs the features contained in master:
	- updated video interface
	- keyboard driver

Conflicts:
	src/Makefile
	src/main.asm
	src/video.asm
updated solution file
updated visual studio solution
simplified some code in hexdigits.asm
implemented page scrolling and address indicator
Fixed scan code tables (F1 was missing)