Build all linux configs by default

- change build-linux to take an array of configs
- build all linux configs defined for a variant
- update all variants to have a base variant that builds everything
- update all sub variants to turn off the bits other than the sub
  category
- fix a few over complications in using variables

Change-Id: I28f212058f966a0020b4fccfaf54abcbdf188f81
Signed-off-by: Mark Hambleton <mark.hambleton@arm.com>

Conflicts:
	build-devtrees.sh
	variants/armstrong-android
	variants/armstrong-busybox
	variants/ashbrook
	variants/buzz-android
	variants/buzz-busybox
	variants/buzz-oe
	variants/css-mobile
	variants/css-mobile-android
	variants/css-mobile-oe
	variants/fvp-android
	variants/fvp-busybox
	variants/fvp-oe
	variants/fvp-uefi
	variants/juno-android
	variants/juno-busybox
	variants/juno-oe
	variants/tc2-android
	variants/tc2-busybox
	variants/tc2-oe