Modify changeReleaseDate to include openwrt release, linux kernel version

[Rangak]

The /etc/openwrt_release file on the router is used to record release info and provides basis for Software Version on Openwireless WebUI Settings screen.

The changeReleaseDate script is used to update this release info at build time thru the sendToBuild script. Currently only the Openwireless git commit hash is auto updated in /etc/openwrt_release. Augment this to auto update openwrt release (r4xxxx) and linux kernel version (3.1x.xx) by reading appropriate files in the build.

Currently we use only the OW git commit to define the timestamp in /etc/last_update_check . It should be set to the latest time from among the three git version stamps i.e. OW, openwrt, linux kernel .