boards: x86_jailhouse: fix build documentation
boards: x86_jailhouse: fix build documentation

The build documentation for this board is misleading. It says to "set
JAILHOUSE_QEMU_IMG_FILE", *then* run cmake commands. But setting the
variable must be part of the cmake commands themselves. Fix it.

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

aceofall/zephyr-iotos,ldts/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,Vudentz/zephyr,punitvara/zephyr,nashif/zephyr,finikorg/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,zephyriot/zephyr,kraj/zephyr,zephyriot/zephyr,mbolivar/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,aceofall/zephyr-iotos,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,explora26/zephyr,explora26/zephyr,Vudentz/zephyr,punitvara/zephyr,kraj/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,mbolivar/zephyr,mbolivar/zephyr,aceofall/zephyr-iotos,kraj/zephyr,aceofall/zephyr-iotos,zephyriot/zephyr,aceofall/zephyr-iotos,zephyriot/zephyr,explora26/zephyr,finikorg/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,nashif/zephyr,mbolivar/zephyr,explora26/zephyr,mbolivar/zephyr,punitvara/zephyr,punitvara/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr