fix(C0): wrong property name

Prevent to redefine build.extra_flags and duplicate
some definitions.

Signed-off-by: Frederic Pillon <frederic.pillon@st.com>

Author: fpistm

boards.txt

@@ -293,7 +293,7 @@ Nucleo_64.menu.pnum.NUCLEO_C031C6.build.series=STM32C0xx
 Nucleo_64.menu.pnum.NUCLEO_C031C6.build.product_line=STM32C031xx
 Nucleo_64.menu.pnum.NUCLEO_C031C6.build.variant=STM32C0xx/C031C(4-6)(T-U)
 Nucleo_64.menu.pnum.NUCLEO_C031C6.build.cmsis_lib_gcc=arm_cortexM0l_math
-Nucleo_64.menu.pnum.NUCLEO_C031C6.build.extra_flags=-D{build.product_line} {build.xSerial} -D__CORTEX_SC=0
+Nucleo_64.menu.pnum.NUCLEO_C031C6.build.st_extra_flags=-D{build.product_line} {build.xSerial} -D__CORTEX_SC=0
 
 # NUCLEO_F030R8 board
 Nucleo_64.menu.pnum.NUCLEO_F030R8=Nucleo F030R8
@@ -908,7 +908,7 @@ Disco.menu.pnum.STM32C0316_DK.build.series=STM32C0xx
 Disco.menu.pnum.STM32C0316_DK.build.product_line=STM32C031xx
 Disco.menu.pnum.STM32C0316_DK.build.variant=STM32C0xx/C031C(4-6)(T-U)
 Disco.menu.pnum.STM32C0316_DK.build.cmsis_lib_gcc=arm_cortexM0l_math
-Disco.menu.pnum.STM32C0316_DK.build.extra_flags=-D{build.product_line} {build.xSerial} -D__CORTEX_SC=0
+Disco.menu.pnum.STM32C0316_DK.build.st_extra_flags=-D{build.product_line} {build.xSerial} -D__CORTEX_SC=0
 
 # DISCO_F030R8 board
 Disco.menu.pnum.DISCO_F030R8=STM32F030R8-DISCVL
@@ -1139,7 +1139,7 @@ GenC0.build.board=GenC0
 GenC0.build.mcu=cortex-m0plus
 GenC0.build.series=STM32C0xx
 GenC0.build.cmsis_lib_gcc=arm_cortexM0l_math
-GenC0.build.extra_flags=-D{build.product_line} {build.xSerial} -D__CORTEX_SC=0
+GenC0.build.st_extra_flags=-D{build.product_line} {build.xSerial} -D__CORTEX_SC=0
 
 # Generic C031C4Tx
 GenC0.menu.pnum.GENERIC_C031C4TX=Generic C031C4Tx