Fix cmake CMAKE_SYSTEM_PROCESSOR string matches

To avoid unwanted matches of regexps, example: "cortex-m3" matches
string "cortex-m33", several string MATCHES have been reworked.

Change-Id: Ib923fbed9edbfba7de0af005c67b5527acdf40f9
diff --git a/rtos/CMakeLists.txt b/rtos/CMakeLists.txt
index 6c91a83..3266c14 100644
--- a/rtos/CMakeLists.txt
+++ b/rtos/CMakeLists.txt
@@ -24,7 +24,7 @@
 
 # Include ThreadX
 # Only enable cortex-m55 until cortex-m33 is sorted out upstream
-if((CMAKE_SYSTEM_PROCESSOR MATCHES "cortex-m55") AND
+if((CMAKE_SYSTEM_PROCESSOR MATCHES "cortex-m55(\\+|$)") AND
     (CORE_SOFTWARE_RTOS_LOWER STREQUAL "threadx" OR CORE_SOFTWARE_RTOS_LOWER STREQUAL "all"))
     #threadx build requires 2 defines: THREADX_ARCH (format: cortex_m4) and THREADX_TOOLCHAIN (ac5/ac6/gnu).
     #set them according to existing defines CMAKE_SYSTEM_PROCESSOR and CMAKE_CXX_COMPILER_ID: