arm: cortex-m: allow configurable ROM offset

Currently, ARM Cortex-M image ROMs are linked starting at the flash
device's base address (CONFIG_FLASH_BASE_ADDRESS). This prevents XIP
Zephyr applications from being linked to run from elsewhere on the
flash device. Linking Zephyr applications to run from elsewhere can be
necessary when running under a bootloader (i.e., booting into a Zephyr
application from a bootloader, not using Zephyr as a bootloader).

To enable this use case, add a new config option: FLASH_LOAD_OFFSET.
This option directs the linker to treat ROM as if it started that many
bytes from the base of flash on Cortex-M targets. The option defaults
to zero to preserve backwards compatibility.

Change-Id: I64f82aee257c19c2451f9789b0ab56999775b761
Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>