Build with bash pipefail even if avra supports pragma directives.
Fix scaling of MIN_RC_PULS (forgot CPU_MHZ multiplicand).

This should make the USB linkers a little less fussy by not arming
unintentionally on short pulses.
Experimental: Rapidly beep at idle when bad PWM input edges are detected.

This should probably stop eventually and may not be ideal as-is, but do
let me know if you see this triggering in normal use.