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>

Makefile

@@ -542,6 +542,11 @@ CONFIG_NI845X_SPI ?= no
 # Disable wiki printing by default. It is only useful if you have wiki access.
 CONFIG_PRINT_WIKI ?= no
 
+# Minimum time in microseconds to suspend execution for (rather than polling)
+# when a delay is required. Larger values may perform better on machines with
+# low timer resolution, at the cost of increased power.
+CONFIG_DELAY_MINIMUM_SLEEP_US ?= 100000
+
 # Disable all features if CONFIG_NOTHING=yes is given unless CONFIG_EVERYTHING was also set
 ifeq ($(CONFIG_NOTHING), yes)
   ifeq ($(CONFIG_EVERYTHING), yes)
@@ -587,6 +592,7 @@ FEATURE_FLAGS += -D'CONFIG_DEFAULT_PROGRAMMER_NAME=NULL'
 endif
 
 FEATURE_FLAGS += -D'CONFIG_DEFAULT_PROGRAMMER_ARGS="$(CONFIG_DEFAULT_PROGRAMMER_ARGS)"'
+FEATURE_FLAGS += -D'CONFIG_DELAY_MINIMUM_SLEEP_US=$(CONFIG_DELAY_MINIMUM_SLEEP_US)'
 
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