misc: Implemented an 'all' mode for the Meson build system

lib/meson.build

@@ -36,6 +36,6 @@ subdir('cm3')
 locm3_ld_script_path = meson.current_source_dir()
 
 # Now take the platform target and map it to a suitable target family
-if target_platform.startswith('stm32')
+if target_platform.startswith('stm32') or target_platform == 'all'
 	subdir('stm32')
 endif

lib/stm32/meson.build

@@ -52,4 +52,10 @@ subdirs = {
 }
 
 # Bring in the proper target subdir for the requested target platform
-subdir(subdirs[target_platform])
+if target_platform != 'all'
+	subdir(subdirs[target_platform])
+else
+	foreach subdir_name, subdir_path : subdirs
+		subdir(subdir_path)
+	endforeach
+endif