conf: machine: imx8m[m,n,p]: enable extlinux support
Newer U-Boot versions enables distro boot option as a default. This
requires that the target uses either Syslinux or boot script in order to
start instead of customized boot environment.
Since according to U-Boot documentation [1], Syslinux is considered as a
preferred way of using distro boot - enable it for imx8mm, imx8mn and
imx8mp derivatives for mainline BSP.
Syslinux options are introduced in machine-specific headers in order
to assemble correct extlinux.conf file.
