openocd.board.cmake: Fix flash load address

'make flash' always erase the flash banks from sector 0, and flashing
the Zephyr.bin file to base address of flash even if there has a
bootloader placed on the first sector.

Users maybe want placed Zephyr to another location instead of sector
0 by adding 'zephyr,code-partition' in dts. But the openocd scripts
doesn't deal with that currently.

Fix this issue, flash zephyr.elf instead of zephyr.bin, elf file
contain load address and openocd scripts can deal with it.

Signed-off-by: qianfan Zhao <qianfanguijin@163.com>