misc: Allow flash image size to be constrained

The CONFIG_FLASH_LOAD_OFFSET allows the zephyr image to be placed at
an offset from the start of flash.  This is useful for situations,
such as with a bootloader, to allow the image to not occupy the very
beginning of flash.

Complement this by adding a CONFIG_FLASH_LOAD_SIZE config, that can
constrain the size of the flash image.  With the default of zero, the
behavior is as before, with the image allowed to occupy the rest of
the flash.  It can also be defined to a non-zero value which will
constrain the image to occupy that many bytes of flash.

Although this is defined generally, it is currently only supported on
cortex-m targets.

Change-Id: I6e4a0e79c8459f931cd4757c932d20dac740f5f6
Signed-off-by: David Brown <david.brown@linaro.org>