Move the revision info to the VERSION_CODE variable and default VERSION_NUMBER
to CURRENT for master branch builds.

Also introduce a new menuconfig option CONFIG_VERSION_CODE which allows users
to override the revision value put into VERSION_CODE and adjust the template
files used by the base-files package to accomodate for the changed semantics.

While we're at it, also adjust the various URLs to match the current web site.

After this commit, the relevent files will look like the examples given below:

    # cat /etc/openwrt_version
    r2398+1

    # cat /etc/openwrt_release
    DISTRIB_ID='LEDE'
    DISTRIB_RELEASE='CURRENT'
    DISTRIB_REVISION='r2398+1'
    DISTRIB_CODENAME='reboot'
    DISTRIB_TARGET='x86/64'
    DISTRIB_DESCRIPTION='LEDE Reboot CURRENT r2398+1'
    DISTRIB_TAINTS='no-all override'

    # cat /usr/lib/os-release
    NAME="LEDE"
    VERSION="CURRENT, Reboot"
    ID="lede"
    ID_LIKE="lede openwrt"
    PRETTY_NAME="LEDE Reboot CURRENT"
    VERSION_ID="current"
    HOME_URL="http://lede-project.org/"
    BUG_URL="http://bugs.lede-project.org/"
    SUPPORT_URL="http://forum.lede-project.org/"
    BUILD_ID="r2398+1"
    LEDE_BOARD="x86/64"
    LEDE_TAINTS="no-all override"
    LEDE_DEVICE_MANUFACTURER="LEDE"
    LEDE_DEVICE_MANUFACTURER_URL="http://lede-project.org/"
    LEDE_DEVICE_PRODUCT="Generic"
    LEDE_DEVICE_REVISION="v0"
    LEDE_RELEASE="LEDE Reboot CURRENT r2398+1"

On a release branch, those files would look like:

    # cat /etc/openwrt_version
    r2399

    # cat /etc/openwrt_release
    DISTRIB_ID='LEDE'
    DISTRIB_RELEASE='16.12-CURRENT'
    DISTRIB_REVISION='r2399'
    DISTRIB_CODENAME='test_release'
    DISTRIB_TARGET='x86/64'
    DISTRIB_DESCRIPTION='LEDE Test Release 16.12-CURRENT r2399'
    DISTRIB_TAINTS='no-all override'

    # cat /usr/lib/os-release
    NAME="LEDE"
    VERSION="16.12-CURRENT, Test Release"
    ID="lede"
    ID_LIKE="lede openwrt"
    PRETTY_NAME="LEDE Test Release 16.12-CURRENT"
    VERSION_ID="16.12-current"
    HOME_URL="http://lede-project.org/"
    BUG_URL="http://bugs.lede-project.org/"
    SUPPORT_URL="http://forum.lede-project.org/"
    BUILD_ID="r2399"
    LEDE_BOARD="x86/64"
    LEDE_TAINTS="no-all override"
    LEDE_DEVICE_MANUFACTURER="LEDE"
    LEDE_DEVICE_MANUFACTURER_URL="http://lede-project.org/"
    LEDE_DEVICE_PRODUCT="Generic"
    LEDE_DEVICE_REVISION="v0"
    LEDE_RELEASE="LEDE Test Release 16.12-CURRENT r2399"

On a release tag, those files would look like:

    # cat /etc/openwrt_version
    r2500

    # cat /etc/openwrt_release
    DISTRIB_ID='LEDE'
    DISTRIB_RELEASE='17.02.1'
    DISTRIB_REVISION='r2500'
    DISTRIB_CODENAME='mighty_unicorn'
    DISTRIB_TARGET='x86/64'
    DISTRIB_DESCRIPTION='LEDE Mighty Unicorn 17.02.1 r2500'
    DISTRIB_TAINTS='no-all override'

    # cat /usr/lib/os-release
    NAME="LEDE"
    VERSION="17.02.1, Mighty Unicorn"
    ID="lede"
    ID_LIKE="lede openwrt"
    PRETTY_NAME="LEDE Mighty Unicorn 17.02.1"
    VERSION_ID="17.02.1"
    HOME_URL="http://lede-project.org/"
    BUG_URL="http://bugs.lede-project.org/"
    SUPPORT_URL="http://forum.lede-project.org/"
    BUILD_ID="r2500"
    LEDE_BOARD="x86/64"
    LEDE_TAINTS="no-all override"
    LEDE_DEVICE_MANUFACTURER="LEDE"
    LEDE_DEVICE_MANUFACTURER_URL="http://lede-project.org/

"
    LEDE_DEVICE_PRODUCT="Generic"
    LEDE_DEVICE_REVISION="v0"
    LEDE_RELEASE="LEDE Mighty Unicorn 17.02.1 r2500"

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
Acked-by: Felix Fietkau <nbd@nbd.name>