fitimage.bbclass: warn if kernel is compressed, but no compression specified

If the kernel build type uses compression, the bootloader needs to take
care of decompression. This must be configured in the FIT image via
FITIMAGE_IMAGE_myimage[comp]. So warn if the FIT image kernel compression
is not specified in such a case.

Signed-off-by: Bastian Krause <bst@pengutronix.de>

meta-oe/classes/fitimage.bbclass

@@ -390,6 +390,8 @@ python write_manifest() {
             imageflags = d.getVarFlags('FITIMAGE_IMAGE_%s' % image, expand=['file', 'fstype', 'type', 'comp']) or {}
             imgtype = imageflags.get('type', 'kernel')
             if imgtype == 'kernel':
+                if d.getVar('KERNEL_IMAGETYPE') not in ('zImage', 'Image') and not imageflags.get('comp'):
+                    bb.warn(f"KERNEL_IMAGETYPE is '{d.getVar('KERNEL_IMAGETYPE')}' but FITIMAGE_IMAGE_kernel[comp] is not set.")
                 default = "%s-%s%s" % (d.getVar('KERNEL_IMAGETYPE'), machine, d.getVar('KERNEL_IMAGE_BIN_EXT'))
                 imgsource = imageflags.get('file', default)
                 imgcomp = imageflags.get('comp', 'none')