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>

lib/hashtable.c

diff --git a/lib/hashtable.c b/lib/hashtable.c
index 19d5b15..026dbca 100644
--- a/lib/hashtable.c
+++ b/lib/hashtable.c
@@ -155,7 +155,7 @@
 		if (htab->table[i].used > 0) {
 			ENTRY *ep = &htab->table[i].entry;
 
-			free(ep->key);
+			free((void *)ep->key);
 			free(ep->data);
 		}
 	}
@@ -416,7 +416,7 @@
 	/* free used ENTRY */
 	debug("hdelete: DELETING key \"%s\"\n", key);
 
-	free(ep->key);
+	free((void *)ep->key);
 	free(ep->data);
 	htab->table[idx].used = -1;
 
@@ -564,7 +564,7 @@
 	 * export sorted list of result data
 	 */
 	for (i = 0, p = res; i < n; ++i) {
-		char *s;
+		const char *s;
 
 		s = list[i]->key;
 		while (*s)