[coreboot] setting ipxe rom path deslects ipxe option

[Andrew Engelbrecht]

when using "make menuconfig", i selected "add pxe rom" then i selected
the field below to set the path of the pxe rom image. i entered the
path, then pressed enter. as a result, the "add pxe rom" option was
disabled. i then re-enabled that option and saw the path that i entered.

if not paying attention, one might think they've enabled pxe, but fail
to enable it. i think this is likely a bug. i expected the pxe option to
not be disabled by entering a pxe image path.

andrew