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/common.h b/include/common.h
index 1572791..b994e70 100644
--- a/include/common.h
+++ b/include/common.h
@@ -262,13 +262,13 @@
 int	env_init     (void);
 void	env_relocate (void);
 int	envmatch     (uchar *, int);
-char	*getenv	     (char *);
-int	getenv_f     (char *name, char *buf, unsigned len);
+char	*getenv	     (const char *);
+int	getenv_f     (const char *name, char *buf, unsigned len);
 int	saveenv	     (void);
 #ifdef CONFIG_PPC		/* ARM version to be fixed! */
-int inline setenv   (char *, char *);
+int inline setenv    (const char *, const char *);
 #else
-int	setenv	     (char *, char *);
+int	setenv	     (const char *, const char *);
 #endif /* CONFIG_PPC */
 #ifdef CONFIG_ARM
 # include <asm/mach-types.h>