kconfig: allow specifying the seed for randconfig

For reproducibility, it can be useful to be able to specify the
seed to use to seed the RNG.

Add a new KCONFIG_SEED environment variable which can be set to
the seed to use:
    $ make KCONFIG_SEED=42 randconfig
    $ sha1sum .config
    70a128c8dcc61303069e1be352cce64114dfcbca  .config
    $ make KCONFIG_SEED=42 randconfig
    $ sha1sum .config
    70a128c8dcc61303069e1be352cce64114dfcbca  .config

It's very usefull for eg. debugging the kconfig parser.
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

Documentation/kbuild/kconfig.txt

@@ -89,6 +89,15 @@ These examples will disable most options (allnoconfig) but enable or
 disable the options that are explicitly listed in the specified
 mini-config files.
 
+______________________________________________________________________
+Environment variables for 'randconfig'
+
+KCONFIG_SEED
+--------------------------------------------------
+You can set this to the integer value used to seed the RNG, if you want
+to somehow debug the behaviour of the kconfig parser/frontends.
+If not set, the current time will be used.
+
 ______________________________________________________________________
 Environment variables for 'silentoldconfig'
 

scripts/kconfig/conf.c

@@ -13,6 +13,7 @@
 #include <getopt.h>
 #include <sys/stat.h>
 #include <sys/time.h>
+#include <errno.h>
 
 #include "lkc.h"
 
@@ -514,14 +515,23 @@ int main(int ac, char **av)
 		{
 			struct timeval now;
 			unsigned int seed;
+			char *seed_env;
 
 			/*
 			 * Use microseconds derived seed,
 			 * compensate for systems where it may be zero
 			 */
 			gettimeofday(&now, NULL);
-
 			seed = (unsigned int)((now.tv_sec + 1) * (now.tv_usec + 1));
+
+			seed_env = getenv("KCONFIG_SEED");
+			if( seed_env && *seed_env ) {
+				char *endp;
+				int tmp = (int)strtol(seed_env, &endp, 10);
+				if (*endp == '\0') {
+					seed = tmp;
+				}
+			}
 			srand(seed);
 			break;
 		}