Build system: Add multi-platform support

Move all explicit platform or architecture specific references
into a new platform.mk file that is defined for each platform.

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
Build system: minor spacing tidyup

Tidy up the spacing of variable definitions within the makefiles to make
them more consistent, easier to read and amend.

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>