scripts: jlink: Add flash command support to the jlink runner

Adds support for the 'flash' command to the jlink runner so we can flash
via 'make flash'. This works by generating a temporary jlink commander
script file and passing the script to the jlink commander executable.

Previously, the only way to flash with jlink was via 'make debug'.

Includes support for DT based flash address, and an optional mass erase
before loading the flash.

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