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/exports.h b/include/exports.h
index e14d727..9492566 100644
--- a/include/exports.h
+++ b/include/exports.h
@@ -21,8 +21,8 @@
 int vprintf(const char *, va_list);
 unsigned long simple_strtoul(const char *cp,char **endp,unsigned int base);
 int strict_strtoul(const char *cp, unsigned int base, unsigned long *res);
-char *getenv (char *name);
-int setenv (char *varname, char *varvalue);
+char *getenv (const char *name);
+int setenv (const char *varname, const char *varvalue);
 long simple_strtol(const char *cp,char **endp,unsigned int base);
 int strcmp(const char * cs,const char * ct);
 int ustrtoul(const char *cp, char **endp, unsigned int base);