udelay: only use OS time for delays, except on DOS

As proposed on the mailing list ("RFC: remove the calibrated delay
loop"), this removes the calibrated delay loop and uses OS-based timing
functions for all delays because the calibrated delay loop can delay for
shorter times than intended.

When sleeping this now uses nanosleep() if available, and otherwise
usleep(). When busy-looping, it uses clock_gettime() with
CLOCK_MONOTONIC or CLOCK_REALTIME depending on availability, and
gettimeofday() otherwise.

The calibrated delay loop is retained for DOS only, because timer
resolution on DJGPP is only about 50 milliseconds. Since typical delays
in flashrom are around 10 microseconds, using OS timing there would
regress performance by around 500x. The old implementation is reused
with some branches removed based on the knowledge that timer resolution
will not be better than about 50 milliseconds.

Signed-off-by: Peter Marheine <pmarheine@chromium.org>
Change-Id: I7ac5450d194a475143698d65d64d8bcd2fd25e3f
---
M Makefile
M include/programmer.h
M libflashrom.c
M meson.build
M udelay.c
A udelay_dos.c
6 files changed, 203 insertions(+), 171 deletions(-)