Build: Refactor toolchain files

Change from a CMAKE toolchain file to a TFM toolchain file, avoiding
some abuses of the CMAKE_TOOLCHAIN_FILE that were used as a workaround
for compiler setup. Also add the CROSS_COMPILE variable. Bump cmake
required version to 3.15.

Change-Id: I0948033045e2d2f34beffa807925fc7375098335
Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/docs/getting_started/tfm_sw_requirement.rst b/docs/getting_started/tfm_sw_requirement.rst
index 24d42aa..9e83d23 100644
--- a/docs/getting_started/tfm_sw_requirement.rst
+++ b/docs/getting_started/tfm_sw_requirement.rst
@@ -81,12 +81,7 @@
 
 The build-system is CMake based and supports the following versions:
 
-    - 3.13
-    - 3.14
-    - 3.15
-    - 3.16
-    - 3.17
-    - 3.18
+The build-system is CMake based and supports the versions 3.15 or higher.
 
 .. Note::
     - IAR requires version 3.14 or later.