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>

common/bootm.c

@@ -569,7 +569,7 @@ static void fixup_silent_linux(void)
 		env_val = CONSOLE_ARG;
 	}
 
-	setenv("bootargs", env_val);
+	env_set("bootargs", env_val);
 	debug("after silent fix-up: %s\n", env_val);
 	free(buf);
 }