refactor(build): define the W and DEBUG flags in the standard way

We have the assert_boolean section where DEBUG belongs and W should have
an initial value (of 0) and only be allowed to be a number.

Change-Id: Ie56dc9659f32c8a202f4506bc27e8bbf84c0f73f
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
2 files changed