[buildroot] include, base-files, opkg: introduce version configuration to override...

[buildroot] include, base-files, opkg: introduce version configuration to override the embedded version info of generated images
 - Introduce new Kconfig symbols VERSION_DIST, VERSION_NICK, VERSION_NUMBER and VERSION_REPO to specify distribution, release name, version and repository for a given build
 - Introduce include/version.mk to provide common helpers for packages dealing with versions
 - Make opkg use version.mk to populate the opkg.conf template
 - Make base-files use version.mk to populate /etc/openwrt_version, /etc/openwrt_release and /etc/banner

The available placeholders are:
 %D .. Replace with $(CONFIG_VERSION_DIST), default to "OpenWrt"
 %d .. Like %D, but all characters made lowercase and spaces substituted with "_" (e.g. "openwrt")
 %N .. Replace with $(CONFIG_VERSION_NICK), default to the build tree release (e.g. "Attitude Adjustment")
 %n .. Like %N, but all characters made lowercase and spaces substituted with "_" (e.g. "attitude_adjustment")
 %V .. Replace with $(CONFIG_VERSION_NUMBER), default to the build tree release (e.g. "r31262")
 %v .. Like %V, but all characters made lowercase and spaces substituted with "_"
 %C .. Replace with $(CONFIG_VERSION_NUMBER), default to "Bleeding Edge"
 %c .. Like %C, but all characters made lowercase and spaces substituted with "_" (e.g. "bleeding_edge")
 %U .. Replace with $(CONFIG_VERSION_REPO), default to "http://downloads.openwrt.org/snapshots/trunk/%T/packages"
 %R .. Replace with the current build tree revision (e.g. "r31262" or "75488c4a05b8033cf69e91874a61852db7ba9c6c")
 %T .. Replace with the current target (e.g. "ar71xx")
 %S .. Replace with the current target/subtarget combo (e.g. "adm5120/router_le")

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@31262 3c298f89-4303-0410-b956-a3cf2f4a3e73