Make sleep threshold for delays configurable

This allows the minimum time that default_delay() will choose to sleep
for instead of polling to be configured at build-time. The default
remains unchanged at 100 milliseconds for now.

The test's correctness has been checked by testing with minimum sleep
time left at its default and set to a non-default value smaller than 100
microseconds (both pass without sleeping, verified with strace) and with
the minimum sleep time set to 0 (causing the test to be skipped). The
configured value from the macro needs to be stored in a const to avoid
-Werror=type-limits errors when configured to be zero.

Change-Id: Ida96e0816ac914ed69d6fd82ad90ebe89cdef1cc
Signed-off-by: Peter Marheine <pmarheine@chromium.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/81606
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>

meson.build

@@ -112,6 +112,9 @@ if target_machine.system() == 'dos'
   delay_src = files('udelay_dos.c')
 endif
 srcs += delay_src
+cargs += ['-DCONFIG_DELAY_MINIMUM_SLEEP_US=@0@'.format(
+  get_option('delay_minimum_sleep_us')
+)]
 
 # check for required symbols
 if cc.has_function('clock_gettime')