subdirmk/generate: Supply default target if none given in command line

subdirmk/generate: Supply default target if none given in command line

If you just run `make' then `$(MAKECMDGOALS)' is empty, so we get the
default target of `main.mk'.  Nothing is set up by `usual.mk', so we
end up with `$(srcdir)/configure' established by `regen.mk'.

Instead, propagate `all' to the `main.mk' makefile.  Maybe this should
be configurable, though I don't think that'll be very useful in real
life.

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>