u-boot: state the MACHINE when skipping u-boot

u-boot: state the MACHINE when skipping u-boot

If the user accidently tries building u-boot on a machine doesn't use u-boot
(such as qemuarm) the error message doesn't make it clear why u-boot was
skipped.  To help, state the machine that was being built for again.

[ YOCTO #4945 ]

(From OE-Core rev: beef66beaee926ec3d3640b79133fdb2ccc404f0)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>