platform/common: keep u-boot build disabled by default

u-boot build is not required for SGI platforms. So, u-boot build
can be marked as disabled by default and platforms that need
the u-boot stage should enable it.

Change-Id: I45f8da49612e7578c27b3174cc4f791e17453b33
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>