Make bob_defaults an independent module type
Make bob_defaults an independent module type

Rather than just embedding `library`, give defaults its own properties
structure and interface implementation methods. The properties are still
just an embedded version of `BuildProps`, but this will allow a future
commit to change that, paving the way for a future separation of e.g.
kernel module and library/binary properties.

Change-Id: I7741c61af17bb1a6649985152d6060dfaddf9b56
Signed-off-by: Chris Diamand <1f58517798882095082bdee11eba81705511491e@arm.com>

ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build