Build: Change default build option to MinSizeRel

* By default CMake's Release option builds with -O3,
  which isn't efficient for Cortex-M devices,
  where we wanted the lowest ROM/RAM usage.
* If TFM_SYSTEM_PROCESSOR is defined, then build and link
  with this cpu instead of arch with both
  armclang and gcc

Signed-off-by: Dávid Házi <david.hazi@arm.com>
Change-Id: I7550ab435fcf6bb595dcc153665009e47163214b
diff --git a/docs/building/tfm_build_instruction.rst b/docs/building/tfm_build_instruction.rst
index 6d9f50a..62521ff 100644
--- a/docs/building/tfm_build_instruction.rst
+++ b/docs/building/tfm_build_instruction.rst
@@ -96,7 +96,7 @@
 Build type
 ----------
 
-By default, a release configuration is built. Alternate build types can be
+By default, a MinSizeRel configuration is built. Alternate build types can be
 specified with the ``CMAKE_BUILD_TYPE`` variable. The possible
 types are:
 
@@ -105,7 +105,7 @@
  - ``Release``
  - ``MinSizeRel``
 
-``Release`` is default.
+``MinSizeRel`` is default.
 
 Debug symbols are added by default to all builds, but can be removed
 from ``Release`` and ``MinSizeRel`` builds by setting