In an effort to avoid unexpected behaviour from users not calibrating
BOARD_LOOPSPERMSEC, a compilation error occurs when it is undefined. On
some systems, this is allowed (those that use timer implementations
instead of busy-looping for delays). In these cases, we should busy-loop
using the clock time instead of relying on a possibly
undefined/uncalibrated macro.
Signed-off-by: Matteo Golin <matteo.golin@gmail.com>