boot: Improve comments related to global bootmeths

Add a few comments about global bootmeths and first_glob_method

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@kernel.org>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>

boot/bootflow.c

@@ -344,6 +344,7 @@ static int bootflow_check(struct bootflow_iter *iter, struct bootflow *bflow)
 	struct udevice *dev;
 	int ret;
 
+	/* handle global bootmeths if needed */
 	if (IS_ENABLED(CONFIG_BOOTMETH_GLOBAL) && iter->doing_global) {
 		bootflow_iter_set_dev(iter, NULL, 0);
 		ret = bootmeth_get_bootflow(iter->method, bflow);