Allow use of a custom peripheral pins file

Syntax in the board.txt:
xxx.build.peripheral_pins=-DCUSTOM_PERIPHERAL_PINS

Example to use a custom peripheral pins file for Nucleo_L476RG:
    Nucleo_64.menu.pnum.NUCLEO_L476RG.build.peripheral_pins=-DCUSTOM_PERIPHERAL_PINS
Then add a file with ".c" extension like "PeripheralPins_<board>.c" in the
variantfolder.

At sketch folder:
CUSTOM_PERIPHERAL_PINS could now be overridden using the build_opt.h, ex:
-DCUSTOM_PERIPHERAL_PINS=\"PeripheralPins_NUCLEO_L476RG.c\"

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

Author: fpistm

platform.txt

@@ -55,7 +55,7 @@ build.bootloader_flags=
 build.ldscript=ldscript.ld
 
 # These can be overridden in platform.local.txt
-compiler.c.extra_flags=
+compiler.c.extra_flags={build.peripheral_pins}
 compiler.c.elf.extra_flags=
 compiler.cpp.extra_flags=
 compiler.cpp.std=gnu++14
@@ -96,6 +96,7 @@ build.xSerial=-DHAL_UART_MODULE_ENABLED
 build.enable_usb=
 build.usb_speed=
 build.enable_virtio=
+build.peripheral_pins=
 build.startup_file=
 build.flags.fp=
 build.flags.optimize=-Os