kernel-uboot: allow compression option to be configurable

While some platforms might choose to compress the kernel by
default especially when boot medium is slow, others want
uncompressed kernel.

The choice of decompression speed vs. load speed is very
platform dependent.

Allow platform to choose the option here.

(From OE-Core rev: 5c72105e2973e613b5c0f0e6310ffdea6e56c6c7)

Signed-off-by: Sinan Kaya <okaya@kernel.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/kernel-uboot.bbclass

@@ -1,3 +1,7 @@
+# fitImage kernel compression algorithm
+FIT_KERNEL_COMP_ALG ?= "gzip"
+FIT_KERNEL_COMP_ALG_EXTENSION ?= ".gz"
+
 uboot_prep_kimage() {
 	if [ -e arch/${ARCH}/boot/compressed/vmlinux ]; then
 		vmlinux_path="arch/${ARCH}/boot/compressed/vmlinux"
@@ -11,8 +15,8 @@ uboot_prep_kimage() {
 		linux_comp="none"
 	else
 		vmlinux_path="vmlinux"
-		linux_suffix=".gz"
-		linux_comp="gzip"
+		linux_suffix="${FIT_KERNEL_COMP_ALG_EXTENSION}"
+		linux_comp="${FIT_KERNEL_COMP_ALG}"
 	fi
 
 	[ -n "${vmlinux_path}" ] && ${OBJCOPY} -O binary -R .note -R .comment -S "${vmlinux_path}" linux.bin