fix calculation of nrf firmware size

[dhalbert]

@xobs thanks for informal review in #2789 (comment).

CIRCUITPY_FIRMWARE_SIZE needs to be calculated based on the start address of the region just above it.

circuitpython/ports/nrf/boards/common.template.ld

Lines 8 to 18 in 51dbe91

	FLASH (rx) : ORIGIN = 0x00000000, LENGTH = ${FLASH_SIZE} /* entire flash */
	/* nRF SoftDevice */
	FLASH_MBR (rx) : ORIGIN = ${MBR_START_ADDR}, LENGTH = ${MBR_SIZE}
	FLASH_SD (rx) : ORIGIN = ${SD_FLASH_START_ADDR}, LENGTH = ${SD_FLASH_SIZE}
	FLASH_ISR (rx) : ORIGIN = ${ISR_START_ADDR}, LENGTH = ${ISR_SIZE}
	FLASH_FIRMWARE (rx) : ORIGIN = ${CIRCUITPY_FIRMWARE_START_ADDR}, LENGTH = ${CIRCUITPY_FIRMWARE_SIZE}
	FLASH_BLE_CONFIG (r) : ORIGIN = ${CIRCUITPY_BLE_CONFIG_START_ADDR}, LENGTH = ${CIRCUITPY_BLE_CONFIG_SIZE}
	FLASH_NVM (r) : ORIGIN = ${CIRCUITPY_INTERNAL_NVM_START_ADDR}, LENGTH = ${CIRCUITPY_INTERNAL_NVM_SIZE}
	FLASH_FATFS (r) : ORIGIN = ${CIRCUITPY_INTERNAL_FLASH_FILESYSTEM_START_ADDR}, LENGTH = ${CIRCUITPY_INTERNAL_FLASH_FILESYSTEM_SIZE}
	FLASH_BOOTLOADER (rx) : ORIGIN = ${BOOTLOADER_START_ADDR}, LENGTH = ${BOOTLOADER_SIZE}
	FLASH_BOOTLOADER_SETTINGS (r) : ORIGIN = ${BOOTLOADER_SETTINGS_START_ADDR}, LENGTH = ${BOOTLOADER_SETTINGS_SIZE}

The original calculation before #2789 is wrong because it didn't account for FLASH_BLE_CONFIG and FLASH_NVM, but the #2789 change didn't account for the size of FLASH_NVM.

[tannewt]

Thanks for the fix!