ip806x: nbg6817: don't hardcode the rootfs location by using append-rootblock instead

ip806x: nbg6817: don't hardcode the rootfs location by using append-rootblock instead

This changes the cmdline from:

Kernel command line: root=/dev/mmcblk0p5 rootfstype=squashfs,ext4 rootwait noinitrd
Bootloader command line (ignored): board=NBG6817 root=/dev/mmcblk0p5 rootwait zld_ver=2.04 console=ttyHSL1,115200n8 mtdparts=m25p80:0xC0000(SBL)ro,0x40000(TZ)ro,0x40000(RPM)ro,0x80000(u-boot)ro,0x10000(env)ro,0x10000(ART)ro,0x10000(dualflag),0x210000(reserved)

to

Kernel command line: rootfstype=squashfs,ext4 rootwait noinitrd root=/dev/mmcblk0p5
Bootloader command line (ignored): board=NBG6817 root=/dev/mmcblk0p5 rootwait zld_ver=2.04 console=ttyHSL1,115200n8 mtdparts=m25p80:0xC0000(SBL)ro,0x40000(TZ)ro,0x40000(RPM)ro,0x80000(u-boot)ro,0x10000(env)ro,0x10000(ART)ro,0x10000(dualflag),0x210000(reserved)

As a consequence booting from the alternative dual-boot partition set
(root=/dev/mmcblk0p8) becomes possible.

Signed-off-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>