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>

cmd/md5sum.c

@@ -35,7 +35,7 @@ static void store_result(const u8 *sum, const char *dest)
 			sprintf(str_ptr, "%02x", sum[i]);
 			str_ptr += 2;
 		}
-		setenv(dest, str_output);
+		env_set(dest, str_output);
 	}
 }