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>

udelay.c

@@ -92,14 +92,15 @@ void internal_sleep(unsigned int usecs)
 #endif
 }
 
+static const unsigned min_sleep = CONFIG_DELAY_MINIMUM_SLEEP_US;
+
 /* Precise delay. */
 void default_delay(unsigned int usecs)
 {
-	/* If the delay is >0.1 s, use internal_sleep because timing does not need to be so precise. */
-	if (usecs > 100000) {
-		internal_sleep(usecs);
-	} else {
+	if (usecs < min_sleep) {
 		clock_usec_delay(usecs);
+	} else {
+		internal_sleep(usecs);
 	}
 }