mkimage: Disable build on darwin This will need a lot more work to enable. U-Boot has copies of the <asm/> and <linux/> headers and they aren't compatible with the Darwin stdlib versions. Bug: 74437007 Change-Id: I2b94e94b2f74b7b0acc7e4804e2f85530cab3174 Signed-off-by: Alistair Strachan <[email protected]>