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_secure_boot.rst b/docs/getting_started/tfm_secure_boot.rst
index f9f4599..61be329 100644
--- a/docs/getting_started/tfm_secure_boot.rst
+++ b/docs/getting_started/tfm_secure_boot.rst
@@ -309,7 +309,7 @@
 
 Example of how to provide the secure image minimum version::
 
-    cmake -DTFM_PLATFORM=musca_b1 -DCMAKE_TOOLCHAIN_FILE=../toolchain_GNUARM.cmake -DMCUBOOT_S_IMAGE_MIN_VER=1.2.3+4 ..
+    cmake -DTFM_PLATFORM=musca_b1 -DTFM_TOOLCHAIN_FILE=../toolchain_GNUARM.cmake -DMCUBOOT_S_IMAGE_MIN_VER=1.2.3+4 ..
 
 ********************
 Signature algorithms
@@ -428,7 +428,7 @@
 The version number of the image (single image boot) can manually be passed in
 through the command line in the cmake configuration step::
 
-    cmake -DTFM_PLATFORM=musca_b1 -DCMAKE_TOOLCHAIN_FILE=../toolchain_GNUARM.cmake -DIMAGE_VERSION_S=1.2.3+4 ..
+    cmake -DTFM_PLATFORM=musca_b1 -DTFM_TOOLCHAIN_FILE=../toolchain_GNUARM.cmake -DIMAGE_VERSION_S=1.2.3+4 ..
 
 Alternatively, the version number can be less specific (e.g 1, 1.2, or 1.2.3),
 where the missing numbers are automatically set to zero. The image version
@@ -462,7 +462,7 @@
 current image version. The value of the security counter (single image boot) can
 be specified at build time in the cmake configuration step::
 
-    cmake -DTFM_PLATFORM=musca_b1 -DCMAKE_TOOLCHAIN_FILE=../toolchain_GNUARM.cmake -DSECURITY_COUNTER_S=42 ../
+    cmake -DTFM_PLATFORM=musca_b1 -DTFM_TOOLCHAIN_FILE=../toolchain_GNUARM.cmake -DSECURITY_COUNTER_S=42 ../
 
 The security counter can be independent from the image version, but not
 necessarily. Alternatively, if it is not specified at build time with the