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/design_documents/profiles/tfm_profile_medium.rst b/docs/design_documents/profiles/tfm_profile_medium.rst
index cb744d4..8413635 100644
--- a/docs/design_documents/profiles/tfm_profile_medium.rst
+++ b/docs/design_documents/profiles/tfm_profile_medium.rst
@@ -419,7 +419,7 @@
    cd <TFM root dir>
    mkdir build && cd build
    cmake -DTFM_PLATFORM=mps2/an521 \
-         -DCMAKE_TOOLCHAIN_FILE=../toolchain_ARMCLANG.cmake \
+         -DTFM_TOOLCHAIN_FILE=../toolchain_ARMCLANG.cmake \
          -DTFM_PROFILE=profile_medium \
          -DCMAKE_BUILD_TYPE=MinSizeRel \
          ../
@@ -433,7 +433,7 @@
    cd <TFM root dir>
    mkdir build && cd build
    cmake -DTFM_PLATFORM=mps2/an521 \
-         -DCMAKE_TOOLCHAIN_FILE=../toolchain_ARMCLANG.cmake \
+         -DTFM_TOOLCHAIN_FILE=../toolchain_ARMCLANG.cmake \
          -DTFM_PROFILE=profile_medium \
          -DCMAKE_BUILD_TYPE=MinSizeRel \
          -DTEST_S=ON -DTEST_NS=ON \