Allow for easy change of the WLAN password

[wellenvogel]

Currently changig the wlan password requires to edit cfg files in the system partition. This is rather difficult for people not being familiar with linux.
So there should be an easy way to do this.
There could be a file in the boot partition that will be included in the hostapd conf.
This can easily be edited before starting at all.

[wellenvogel]

with release-20170410 you can use a file /boot/hostapd.conf to set some global values for all hostapd runs.
This especially allows to e.g. set
wpa_passphrase=myveryspecialpassword

see also https://www.segeln-forum.de/board1-rund-ums-segeln/board35-bootstechnik-und-elektronik/43757-raspberry-pi-als-bordcomputer-spielzeug-f%C3%BCr-den-winter/index28.html#post1733731