[Feature]Add support for custom bootloader offset on arduino core

[VitorBoss]

Recently I'm working on a project which will use sboot_stm32 encryption, the current problem is the linker script uses LD_FLASH_OFFSET but I can't set this flag from PlatformIO, I had tried put the file "platform.local.txt" with the build.flash_offset=0x2000 definition, the -DLD_FLASH_OFFSET=0x2000 on compilation flags and on file "build_opt.h" as the Arduino_Core_STM32 wiki says.

The problem is, don't matter what I do, the offset never get applied and the elf file always point to 0x08000000 as starting address. What I'm missing?