bs: Rename MODE_DEBUG to BUILD_MODE_DEBUG
Created by: nicola-mazzucato-arm
Renamed as all symbols/define exposed by the build system should have the prefix BUILD_ in their names.
Change-Id: I8893121b32c31584a163a991547925cc028ee1c9 Signed-off-by: Nicola Mazzucato nicola.mazzucato@arm.com