The object file should not be built inside the submodule, as that can
confuse git.
Not everything depends on the libbase58 header (CCAN doesn't), so
move that to the everything-else depends line.
The BITCOIN_SRC etc should also move to bitcoin/Makefile, but that's
a bigger change.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>