add doc for building examples with cmake build system

change ci matrix build:
- github build make/cmake one per family on push only
- circicle do full cmake build for all toolchain (missing rx-gcc)

hw/bsp/msp430/family.cmake

@@ -11,36 +11,37 @@ set(CMAKE_TOOLCHAIN_FILE ${TOP}/examples/build_system/cmake/toolchain/msp430_${T
 
 set(FAMILY_MCUS MSP430x5xx CACHE INTERNAL "")
 
-
 #------------------------------------
 # BOARD_TARGET
 #------------------------------------
 # only need to be built ONCE for all examples
 function(add_board_target BOARD_TARGET)
-  if (NOT TARGET ${BOARD_TARGET})
-    add_library(${BOARD_TARGET} INTERFACE)
-    target_compile_definitions(${BOARD_TARGET} INTERFACE
-      CFG_TUD_ENDPOINT0_SIZE=8
-      CFG_EXAMPLE_VIDEO_READONLY
-      CFG_EXAMPLE_MSC_READONLY
-      )
-    target_include_directories(${BOARD_TARGET} INTERFACE
-      ${CMAKE_CURRENT_FUNCTION_LIST_DIR}
-      ${SDK_DIR}
-      )
+  if (TARGET ${BOARD_TARGET})
+    return()
+  endif ()
 
-    update_board(${BOARD_TARGET})
+  add_library(${BOARD_TARGET} INTERFACE)
+  target_compile_definitions(${BOARD_TARGET} INTERFACE
+    CFG_TUD_ENDPOINT0_SIZE=8
+    CFG_EXAMPLE_VIDEO_READONLY
+    CFG_EXAMPLE_MSC_READONLY
+    )
+  target_include_directories(${BOARD_TARGET} INTERFACE
+    ${CMAKE_CURRENT_FUNCTION_LIST_DIR}
+    ${SDK_DIR}
+    )
 
-    if (CMAKE_C_COMPILER_ID STREQUAL "GNU")
-      target_link_options(${BOARD_TARGET} INTERFACE
-        "LINKER:--script=${LD_FILE_GNU}"
-        -L${SDK_DIR}
-        )
-    elseif (CMAKE_C_COMPILER_ID STREQUAL "IAR")
-      target_link_options(${BOARD_TARGET} INTERFACE
-        "LINKER:--config=${LD_FILE_IAR}"
-        )
-    endif ()
+  update_board(${BOARD_TARGET})
+
+  if (CMAKE_C_COMPILER_ID STREQUAL "GNU")
+    target_link_options(${BOARD_TARGET} INTERFACE
+      "LINKER:--script=${LD_FILE_GNU}"
+      -L${SDK_DIR}
+      )
+  elseif (CMAKE_C_COMPILER_ID STREQUAL "IAR")
+    target_link_options(${BOARD_TARGET} INTERFACE
+      "LINKER:--config=${LD_FILE_IAR}"
+      )
   endif ()
 endfunction()
 
@@ -75,7 +76,6 @@ function(family_configure_example TARGET RTOS)
     )
   target_link_libraries(${TARGET} PUBLIC board_${BOARD})
 
-
   # Flashing
   family_add_bin_hex(${TARGET})
   family_flash_msp430flasher(${TARGET})