vexpress: Tidy up config so it's clearer and more maintainable

Give the three active projects using the vexpress device their own
cleanly separated sections in BoardConfig.mk and refactor bootloader.mk
so the MP and IKS projects can build the bootloaders they require.

Change-Id: Ib61f68460b2a6b8273168ec82516b8fe99931354
Signed-off-by: Jon Medhurst <tixy@linaro.org>
3 files changed