kbuild: move "_all" target out of $(KBUILD_SRC) conditional

The first "_all" occurrence around line 120 is only visible when
KBUILD_SRC is unset.

If O=... is specified, the working directory is relocated, then the
only second occurrence around line 193 is visible, that is not set
to PHONY.

Move the first one to an always visible place.  This clarifies "_all"
is our default target and it is always set to PHONY.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>