reduce $DEVICE complexity

[Kwiboo]

This PR adds support to use $DEVICE in shell version output and also changes to use same formatting as in the settings addon (<device/project>.<arch>)

Before:

LibreELEC (community): devel-20170711073201-r25890-ge7ffdc769 (Rockchip arm)

After:

LibreELEC (community): devel-20170711073201-r25890-ge7ffdc769 (TinkerBoard.arm)

[MilhouseVH]

No objection to this change in principal, but it could be simplified to:

-echo "$DISTRONAME ($LIBREELEC_BUILD): $LIBREELEC_VERSION ($PROJECT $TARGET_ARCH)" >> $INSTALL/etc/issue 
+echo "$DISTRONAME ($LIBREELEC_BUILD): $LIBREELEC_VERSION (${DEVICE:-${PROJECT}}.$TARGET_ARCH)" >> $INSTALL/etc/issue 

The ${DEVICE:-${PROJECT}} approach could probably be used in several other places and would avoid the more cumbersome - but admittedly more explicit - if..then..else.

Just a matter of style/preference, really.

[Kwiboo]

I like the ${DEVICE:-$PROJECT} approach and have pushed a fixup commit to remove some of the if..then..else, will squash after proper testing

[Kwiboo]

I made a small adjustment and defined LIBREELEC_ARCH="${DEVICE:-$PROJECT}.$TARGET_ARCH" that is reused instead

[Kwiboo]

Squashed and updated to use the ${DEVICE:-$PROJECT} approach in config/path and scripts/create_addon.
Also made scripts/image pass $DEVICE to scripts/mkimage

[MilhouseVH]

Looks good to me, @lrusak any concerns?

[MilhouseVH]

Might also be worth backporting this to libreelec-8.2?