Use Upload Offset in DFU Upload

[maxgerhardt]

The Keyboardio board added in #39 defines one possible upload method for the board as DFU, with an flash_offset of 0x2000.

This is however not respected in the platform.txt DFU upload method. It is respected when building the firmware through relocating the vector table etc. See #39 (comment).

ArduinoCore-GD32/platform.txt

Line 33 in 30bac5f

compiler.c.elf.flags=-mcpu={build.mcu} {build.flags.fp} -mthumb {build.flags.optimize} {build.flags.ldspecs} -Wl,--defsym=LD_FLASH_OFFSET={build.flash_offset} -Wl,--defsym=LD_MAX_SIZE={upload.maximum_size} -Wl,--defsym=LD_MAX_DATA_SIZE={upload.maximum_data_size} -Wl,--cref -Wl,--check-sections -Wl,--gc-sections -Wl,--entry=Reset_Handler -Wl,--unresolved-symbols=report-all -Wl,--warn-common

ArduinoCore-GD32/platform.txt

Lines 110 to 112 in 30bac5f

	tools.serial_upload.upload.params.verbose=-d
	tools.serial_upload.upload.params.quiet=n
	tools.serial_upload.upload.pattern="{path}/{cmd}" {serial.port.file} {upload.altID} {upload.usbID} "{build.path}/{build.project_name}.bin"

We need to very / fix the workings of the DFU upload methods in regards to the upload offset.

Linked to PlatformIO board definition (PR CommunityGD32Cores/platform-gd32#15) too, since DFU upload is also done there but without an upload offset (since the board definition file doesn't have one).