config: xea: Disable support for boot methods EXTLINUX and VBE

The XEA system (imx287 based) is not using support for EXTLINUX and VBE.
As those configuration options have been enabled by default with modern
Kconfig it is safe to explicitly disable them.

After that change the u-boot.img size has been reduced by ~16 KiB.

Signed-off-by: Lukasz Majewski <lukma@denx.de>