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This patch factors out common files required for sp_min for all CSS
platforms from the JUNO specific makefile to a the new `css_sp_min.mk`
makefile. This also allows the common build options that affect CSS
platforms to be configured in a central makefile for sp_min.

Change-Id: Ida952d8833b1aa5eda77ae0a6664a4632aeab24c
Signed-off-by: Soby Mathew <soby.mathew@arm.com>