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/config/config_default.cmake b/config/config_default.cmake
index 017e36e..b795b43 100644
--- a/config/config_default.cmake
+++ b/config/config_default.cmake
@@ -5,6 +5,10 @@
 #
 #-------------------------------------------------------------------------------
 
+set(TFM_TOOLCHAIN_FILE                  ${CMAKE_SOURCE_DIR}/toolchain_GNUARM.cmake CACHE FILEPATH    "Path to TFM compiler toolchain file")
+set(TFM_PLATFORM                        ""          CACHE STRING    "Platform to build TF-M for")
+set(CROSS_COMPILE                       arm-none-eabi CACHE STRING  "Cross-compilation triplet")
+
 set(BL2                                 ON          CACHE BOOL      "Whether to build BL2")
 set(NS                                  ON          CACHE BOOL      "Whether to build NS app")