Makefile: Rename targets for make recursion

We make a few sub-directories recursively, in particular
$(TARGET_DIRS).

For goal "all", we do it the nice way: "all" has a prerequisite
subdir-T for each T in $(TARGET_DIRS), and T's recipe runs make
recursively.  Behaves nicely with -j and -k.

For other goals such as "clean" and "install", the recipe runs make
recursively in a for loop.  Ignores -j and -k.

The next commit will fix that for "clean" and "install".  This commit
prepares the ground by renaming the targets we use for "all" to
include the goal for the sub-make.  This will permit reusing them for
goals other than "all".

Targets subdir-T for T in $(TARGET_DIRS) run "make all" in T.  Rename
to T/all, and declare phony.

Targets romsubdir-R for R in $(ROMS) run "make" in pc-bios/R.  Default
goal is "all" for all R.  Rename to pc-bios/R/all, and declare phony.

The remainder are renamed just for consistency.

Target subdir-dtc runs "make libbft/libfdt.a" in dtc.  Rename to
dtc/all, and declare phony.

Target subdir-capstone runs make $(BUILD_DIR)/capstone/$(LIBCAPSTONE)
in $(SRC_PATH)/capstone.  Rename to capstone/all, and declare phony.

Target subdir-slirp runs "make" in $(SRC_PATH)/slirp.  Default goal is
all, which builds $(BUILD_DIR)/libslirp.a.  Rename to slirp/all, and
declare phony.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190528082308.22032-4-armbru@redhat.com>
[Add compatibility gunk to keep make working across the rename]