src/slave: Clarification to why 8-bit is read into 16 bits like it does.
src/master: operate without DMX (hard-coded).

Some debug progress. turn off dmx and just bang each slave one channel.
if two slaves are banged simultaneously, both of them behave erratically
by sporadic blinking.
src/master: comments.
src/slave: `volatile` qualifier to volatile variables.

Make the slave a bit more volatile just because it has to be quite
volatile while having at least 3 concurrent interrupt handlers ;)
src: hex backups of known-working versions.

add backups of firmware before the ongoing debug session. probably it's
of commit 86ec190
src/slave: Move DMX data read routine from interrupt to main loop.

PCINT3 interrupt disabled. Chan value reading deobfuscated.
src/master: Revert changes.
src/slave: Use LEDs for debugging hang.
src/slave: Better LED debugging.
src/slave: Fix hang in main loop.

Reset the slave-select flag immediately after it changes.
src/master: Send data to slaves continuously. Name LEDs.
src: Move LED definitions to headers.
src/slave: Make interrupts blocking.
src/master: Remove dataisnew initializer, use C99 named struct initialization.
src/master: Better variable names and C99-style declarations.
src/master: Enable C99 mode in Makefile, make functions inline properly.

Inline function definitions had to be moved from spi.c to spi.h,
otherwise they aren't inlined (across translation unit boundaries).
src/master: Remove commented out code.
src: Simplify slave selection.

Instead of toggling slave-select pins, output 1 until slave answers and
then reset back to zero before beginning actual data transmission. This
way the slave doesn't have to track any state and can just wait for a 1.
src: renamed backup files.
Merge remote-tracking branch 'kspi/dev' into dev