Kernel 4.4.6+ on RaspberryPi support .dtbo files for overlays, instead of .dtb.
Add support for both extensions for overlays (".dtb" and ".dtbo")
CAUTION: in IMAGE_CMD_rpi-sdimg, 'TMPDIR' is not set, causing 'STAGING_KERNEL_BUILDDIR' to not be expanded, causing get_dts() to not being able to get the kernel version!
To avoid this problem, the kernel version is added in a new variable outside the scope of IMAGE_CMD_rpi-sdimg, and its value passed to get_dts() and split_overlays()!
Signed-off-by: Herve Jourdain <herve.jourdain@neuf.fr>
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