package/busybox: add comment about variable override

The same comment already exists for UCLIBC_CONFIG_FILE.
Both variables can be override from command-line, as described in the
manual, at section 'Environment variables'.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 01c0fb3862bce6605881d9e32c29b5f7373bb2f9)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/busybox/busybox.mk

@@ -108,6 +108,8 @@ BUSYBOX_MAKE_OPTS = \
 	CONFIG_PREFIX="$(TARGET_DIR)" \
 	SKIP_STRIP=y
 
+# specifying BUSYBOX_CONFIG_FILE on the command-line overrides the .config
+# setting.
 ifndef BUSYBOX_CONFIG_FILE
 BUSYBOX_CONFIG_FILE = $(call qstrip,$(BR2_PACKAGE_BUSYBOX_CONFIG))
 endif