Use official $PROJECT_SRC_DIR

Author: ivankravets

examples/mbed-legacy-examples/mbed-custom-target/platformio.ini

@@ -13,10 +13,10 @@
 platform = ststm32
 framework = mbed
 board = custom_nucleo_l452re_mbed
-board_build.mbed.ldscript = $PROJECTSRC_DIR/TARGET_STM32L452xE/device/TOOLCHAIN_GCC_ARM/STM32L452XX.ld
+board_build.mbed.ldscript = $PROJECT_SRC_DIR/TARGET_STM32L452xE/device/TOOLCHAIN_GCC_ARM/STM32L452XX.ld
 build_flags =
-    -I$PROJECTSRC_DIR/TARGET_STM32L452xE
-    -I$PROJECTSRC_DIR/TARGET_STM32L452xE/device
-    -I$PROJECTSRC_DIR/TARGET_STM32L452xE/TARGET_NUCLEO_L452RE
+    -I$PROJECT_SRC_DIR/TARGET_STM32L452xE
+    -I$PROJECT_SRC_DIR/TARGET_STM32L452xE/device
+    -I$PROJECT_SRC_DIR/TARGET_STM32L452xE/TARGET_NUCLEO_L452RE
 platform_packages =
     framework-mbed @ ~6.51506.0

examples/mbed-rtos-custom-target/platformio.ini

@@ -13,8 +13,8 @@
 platform = ststm32
 framework = mbed
 board = custom_nucleo_l452re_mbed
-board_build.mbed.ldscript = $PROJECTSRC_DIR/TARGET_STM32L452xE/TOOLCHAIN_GCC_ARM/stm32l452xe.ld
-build_flags = 
-    -I$PROJECTSRC_DIR/TARGET_STM32L452xE
-    -I$PROJECTSRC_DIR/TARGET_STM32L452xE/device
-    -I$PROJECTSRC_DIR/TARGET_STM32L452xE/TARGET_NUCLEO_L452RE_P
+board_build.mbed.ldscript = $PROJECT_SRC_DIR/TARGET_STM32L452xE/TOOLCHAIN_GCC_ARM/stm32l452xe.ld
+build_flags =
+    -I$PROJECT_SRC_DIR/TARGET_STM32L452xE
+    -I$PROJECT_SRC_DIR/TARGET_STM32L452xE/device
+    -I$PROJECT_SRC_DIR/TARGET_STM32L452xE/TARGET_NUCLEO_L452RE_P