core: Introduce run() and sleep() callbacks

core: Introduce run() and sleep() callbacks

It seems that the default 'run_avr' function was penalized by
some tests made that were only useful when gdb was active.

This patch introduces two version of the critical run_avr, one for
"raw" running, and one for gdb.

It also introduces a sleep() callback for similar reasons. Ir can
also be used to run unit testing at 100% speed instead of having
random sleep code during execution.

Signed-off-by: Michel Pollet <buserror@gmail.com>