doc: Document E and W build options
doc: Document E and W build options

Change-Id: I0d9dbef7041fcf950bcafcdbbc17c72b4dea9e40
Signed-off-by: Ambroise Vincent <6526170e434f61edb2744ad8311ffdc62335a3f7@arm.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware