mimxrt1050_evk: Add config to select code linking location

Adds a board-level config option to select whether the code gets linked
into internal instruction tightly coupled memory (ITCM), external
hyperflash, or external qspi flash. The default is ITCM.

Note that we are not yet building an image vector table (IVT) into the
image, therefore the zephyr binary is not bootable by the SoC ROM as-is.
However, the DAPLink firmware prepends an IVT to the image when you use
its drag-and-drop feature to program flash, and this results in a
bootable zephyr application.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>