intel-x86-32.conf: add common MACHINE for x86 (32-bit) BOARDS

User need to specify board value to ZEPHYR_BOARD in local.conf
ZEPHYR_BOARD = "minnowboard"

By default it set to MinnowBoard Max 'minnowboard'

Currently 32-bit supported boards:
* up_squared_32
* minnowboard

Ref:
https://docs.zephyrproject.org/latest/boards/x86/index.html



Signed-off-by: Naveen Saini <naveen.kumar.saini@intel.com>