orangepi: drop custom post-build and post-image scripts

Currently in Orange Pi boards post-build script is used only to generate
U-Boot boot script and post-image script is used only to generate sdcard
image according to genimage configuration. However both those tasks can
now be handled by generic Buildroot tools:
- BR2_TARGET_UBOOT_BOOT_SCRIPT config options
- support/scripts/genimage.sh script

This patch drops custom scripts replacing them
by generic Buildroot tools.

Signed-off-by: Sergey Matyukevich <geomatsi@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>