[Skiboot] First 'make check' boot test with simulator pushed

[Stewart Smith]

Hi all,

I pushed a new test case today that uses the POWER8 Simulator (also
known as mambo) to boot skiboot with a tiny payload (rather than a full
linux kernel).

./test/hello_world/run_hello_world.sh

will be run as part of 'make check' and if you have the POWER8 simulator
installed, it will find it and run it (else, the test will be skipped).

My simple test case just makes a single OPAL call to print "Hello
World!", with the source being in test/hello_world/hello_kernel/.