New feature: Run user command or script after OpenWrt is booted

[AlbrechtL]

In commit f7725c1 I added support to run scripts inside OpenWrt after OpenWrt booted up successfully.

Usage see docker-compose.yml

openwrt-docker/docker-compose.yml

Lines 69 to 76 in 07f4eea

	# User command or script to run after OpenWrt is booted
	# Please note that all commands and script will be executed as root
	# If unset -> disabled
	#OPENWRT_AFTER_BOOT_CMD: "/usr/bin/fw_wan_open_http" # Example to open HTTP port 80 at WAN side
	#OPENWRT_AFTER_BOOT_CMD: "/usr/bin/fw_wan_open_http; /usr/bin/fw_wan_open_ssh" # Example to execute two commands
	#OPENWRT_AFTER_BOOT_CMD: "cat /etc/banner" # Custom commands example
	#OPENWRT_AFTER_BOOT_CMD: "a=$$((1+1)); echo The result of 1+1 is $${a}" # Shell script example
	#OPENWRT_AFTER_BOOT_CMD: '[ -f /etc/first_boot_over ] || { touch /etc/first_boot_over; echo "Crazy first boot"; }' # Example to run a command only once

This feature came out from this discussion: #16