configs/orangepi_r1_defconfig: change to GPT partitions for root=PARTLABEL support

configs/orangepi_r1_defconfig: change to GPT partitions for root=PARTLABEL support

GPT provides partition labels, which can be used to tell the kernel to find
the rootfs based on it (root=PARTLABEL=rootfs) as a nicer/more extensible
solution than the MBR disk signature / PARTUUID.

When using GPT, the name of the partition in genimage.cfg is used as the
label for that partition.

The default GPT partition table location unfortunately conflicts with the
SPL location, so move the 16KB GPT table after it.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
[yann.morin.1998@free.fr: add the comment about 'rootfs']
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>