When build variables are assigned or processed en masse, they'd appear
neater in alphabetical order.

Static initializations are moved to a separate file,
make_helpers/defaults.mk, which in itself is sorted alphabetically.

No functional changes.

Change-Id: I966010042b33de6b67592fb9ffcef8fc44d7d128
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>