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 <astrachan@google.com>
1 file changed