Move SET_MACHINE_COMPAT macro to boards.h

Move SET_MACHINE_COMPAT macro to boards.h

pc.h defines a SET_MACHINE_COMPAT macro to make setting up compat_props
for the various PC machine versions less verbose.  There's nothing
inherently PC specific about it, though, so move it to boards.h where other
versioned machine types (like pseries-*) can use it.

While we're doing that, change it's indentation to be a bit more regular.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>