zephyr: fix MCUBOOT_TARGET_CONFIG definition
zephyr: fix MCUBOOT_TARGET_CONFIG definition

When the target doesn't have good enough generic support for MCUBoot,
you can specify the flash layout etc on a per-board basis by creating
a header, boot/zephyr/targets/BOARD.h.

The build system tries to pick this up, but isn't doing it
correctly. Fix it.

Signed-off-by: Marti Bolivar <fd5da480b758a78208111e6ae5c6806e1f887137@opensourcefoundries.com>

runtimeco/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,utzig/mcuboot,utzig/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,utzig/mcuboot,utzig/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,utzig/mcuboot,tamban01/mcuboot,tamban01/mcuboot,runtimeco/mcuboot