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.
Merge remote-tracking branch 'kspi/dev' into dev