kernel: Permit overriding of KERNEL_IMAGETYPE_FOR_MAKE

Commit a1690131691507bbf5853540229b3ad775b836bf removed the ability of recipes to set KERNEL_IMAGETYPE_FOR_MAKE. This can be fixed by only setting KERNEL_IMAGETYPE_FOR_MAKE if it doesn't already have a value. This should hopefully be a good enough approximation to the behaviour prior to that commit to keep such recipes working.

---
 meta/classes/kernel.bbclass | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

index f37affc..73ae7ca 100644
@@ -21,7 +21,8 @@ python __anonymous () {
 
     kerneltype = d.getVar('KERNEL_IMAGETYPE', True)
 
-    d.setVar("KERNEL_IMAGETYPE_FOR_MAKE", re.sub(r'\.gz$', '', kerneltype))
+    if not d.getVar("KERNEL_IMAGETYPE_FOR_MAKE", False):
+        d.setVar("KERNEL_IMAGETYPE_FOR_MAKE", re.sub(r'\.gz$', '', kerneltype))
 
     image = d.getVar('INITRAMFS_IMAGE', True)
     if image:

2.1.4