env: Rename setenv() to env_set()

We are now using an env_ prefix for environment functions. Rename setenv()
for consistency. Also add function comments in common.h.

Suggested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/exports.h b/include/exports.h
index 1d81bc4..6522d78 100644
--- a/include/exports.h
+++ b/include/exports.h
@@ -28,7 +28,7 @@
 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 (const char *name);
-int setenv (const char *varname, const char *varvalue);
+int env_set(const char *varname, const char *value);
 long simple_strtol(const char *cp, char **endp, unsigned int base);
 int strcmp(const char *cs, const char *ct);
 unsigned long ustrtoul(const char *cp, char **endp, unsigned int base);