Constify getenv(), setenv() and hash code functions

This is needed to get rid of build warnings like

main.c:311: warning: passing argument 2 of 'setenv' discards qualifiers from pointer target type

which result from commit 09c2e90 "unify version_string".

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Andreas Bießmann <andreas.devel@googlemail.com>
diff --git a/include/search.h b/include/search.h
index c827d4d..b4edd43 100644
--- a/include/search.h
+++ b/include/search.h
@@ -39,7 +39,7 @@
 } ACTION;
 
 typedef struct entry {
-	char *key;
+	const char *key;
 	char *data;
 } ENTRY;