scripts/image: build an image for each UBOOT_SYSTEM

[Kwiboo]

This PR makes it possible to build multiple images based on the same .kernel and .system files.

When is multiple images built?

Building of multiple images is only activated when BOOTLOADER is u-boot, UBOOT_SYSTEM is empty, DEVICE is set and boards is defined in scripts/uboot_helper.

How is images built?

The u-boot package is built with empty UBOOT_SYSTEM and only update.sh, canupdate.sh and dtbs is installed into .system file at /usr/share/bootloader.
The device universal update <IMAGE_NAME>.tar file is created

For each uboot system returned by scripts/uboot_helper <PROJECT> <DEVICE>:

• u-boot package is re-built and re-installed with UBOOT_SYSTEM set
• bootloader/release is invoked to copy the u-boot files to 3rdparty/bootloader
• scripts/mkimage is invoked and will generate a <IMAGE_NAME>-<UBOOT_SYSTEM>.img.gz image

How does images differ?

The images produced will differ slightly from an image built with UBOOT_SYSTEM set:

• u-boot files installed by bootloader/install is not added to the final image
• using these images to upgrade a device will not upgrade u-boot

Example

Running PROJECT=Rockchip ARCH=arm DEVICE=RK3328 make image will produce:

LibreELEC-RK3328.arm-9.1-devel-20190429010728-13da320-box.img.gz
LibreELEC-RK3328.arm-9.1-devel-20190429010728-13da320-box.img.gz.sha256
LibreELEC-RK3328.arm-9.1-devel-20190429010728-13da320-box-trn9.img.gz
LibreELEC-RK3328.arm-9.1-devel-20190429010728-13da320-box-trn9.img.gz.sha256
LibreELEC-RK3328.arm-9.1-devel-20190429010728-13da320-box-z28.img.gz
LibreELEC-RK3328.arm-9.1-devel-20190429010728-13da320-box-z28.img.gz.sha256
LibreELEC-RK3328.arm-9.1-devel-20190429010728-13da320.kernel
LibreELEC-RK3328.arm-9.1-devel-20190429010728-13da320-roc-cc.img.gz
LibreELEC-RK3328.arm-9.1-devel-20190429010728-13da320-roc-cc.img.gz.sha256
LibreELEC-RK3328.arm-9.1-devel-20190429010728-13da320-rock64.img.gz
LibreELEC-RK3328.arm-9.1-devel-20190429010728-13da320-rock64.img.gz.sha256
LibreELEC-RK3328.arm-9.1-devel-20190429010728-13da320-rockbox.img.gz
LibreELEC-RK3328.arm-9.1-devel-20190429010728-13da320-rockbox.img.gz.sha256
LibreELEC-RK3328.arm-9.1-devel-20190429010728-13da320.system
LibreELEC-RK3328.arm-9.1-devel-20190429010728-13da320.tar
LibreELEC-RK3328.arm-9.1-devel-20190429010728-13da320.tar.sha256

[Kwiboo]

I did an update to remove the requirement of empty UBOOT_VERSION and replaced it with a condition to only build multiple images when scripts/uboot_helper <PROJECT> <DEVICE> return a list of boards.