abootimg/mkboot/mkbootimg classes: Use the newer KERNEL_IMAGE_NAME instead of KERNEL_IMAGE_BASE_NAME and KERNEL_IMAGE_LINK_NAME instead of KERNEL_IMAGE_SYMLINK_NAME on thud

abootimg, mkbootimg, mkboot, statefs, timed: Use the new pkg_postinst_ontarget syntax of OE sumo instead of the previously used failing pkg_postinst

asteroid-image: Define the debug-tweaks IMAGE_FEATURE in asteroid-image.bbclass instead of local.conf

asteroid-image: Always use a fixed 256MB free space (compared to the current 1.3 factor which is the bitbake's default behavior) to make sure there is enough space to save cache/config/logs/alternative watchfaces etc... this makes images slightly larger

Adds an old-kernel-gcc-hdrs bbclass to provide the include/linux/compiler-gccX.h headers needed by 3.10 kernels when built with newer GCC versions (5, 6 and now 7 with the upgrade to rocko)

Move psplash launch from rootfs to initramdisk. Fixes the infinite reboot time, removes a dirty recipe hack and makes the booting experience slightly smoother

asteroid-image: Replace dropbear with openssh to benefit from sftp and its cool qtcreator and gvfs integration