samsung-jxelte: add mainline support

As previously said in the #3072 issue: The patches used on the jxelte device where copied from the exynos7870-mainline repo (maintained by Sergey Lisov) from the 6608cbe5d3b29c44a177d12c14c7f1b070584363 commit.

The dts is based on the dts -I fs -O dts /sys/firmware/base -o exynos7870-j7xelte.dts output of the running downstream PostmarketOS version. And edited in terms of the sleirgoevy device tree repo

With the (at the moment) builded code we can get an invalid kernel length message at startup if no kernel was specify to boot. To avoid this problem I think that we can change the build of the u-boot on the device-samsung-jxelte package. So that the u-boot can be embedded into a image with the kernel as ramdisk. But I will be working on boot the kernel from ITS files or others.

Also the idea was make more universal the linux-postmarketos-exynos7870 package for the rest of Exynos7870 devices. It could make all the device tree files for all Exynos7870 at compiling time and let the u-boot (binary or image) make the rest of the job.

The linux-samsung-jxelte package was updated in terms of binary corrections as kconfig check passes.

Again (both more help but also questions will be welcome) Thank for the Labels and best regards.