Merge #16691: doc: improve depends prefix documentation

2483266 packages.md: document depends build targets (Russell Yanofsky)
be27161 Clarify need to specify --prefix with depends (Russell Yanofsky)

Pull request description:

  There seems to be some confusion about exactly how to use depends, when to pass a prefix to `./configure` etc (see #16367, #16654).

  I've cherry-picked two of russ's commits out of #16367, as they are clear stand-alone improvements and we don't have to wait for #16367 to improve the depends documentation.

ACKs for top commit:
  Sjors:
    utACK 2483266
  hebasto:
    ACK 2483266, I have reviewed the code and it looks OK, I agree it can be merged.
  jonasschnelli:
    ACK 2483266

Tree-SHA512: a198c288248f573519a3b0ef384626b61cc803803280af9a448c28466e3d9949bed0332af6618dac19e81c5a6e9694afa83d976b176fd13c32a6c2c3fea3fc1f

depends/README.md

@@ -12,14 +12,18 @@ For example:
 
     make HOST=x86_64-w64-mingw32 -j4
 
-A prefix will be generated that's suitable for plugging into Bitcoin's
-configure. In the above example, a dir named x86_64-w64-mingw32 will be
+**Bitcoin's configure script by default will ignore the depends output.** In
+order for it to pick up libraries, tools, and settings from the depends build,
+you must point it at the appropriate `--prefix` directory generated by the
+build. In the above example, a prefix dir named x86_64-w64-mingw32 will be
 created. To use it for Bitcoin:
 
     ./configure --prefix=`pwd`/depends/x86_64-w64-mingw32
 
 Common `host-platform-triplets` for cross compilation are:
 
+- `i686-pc-linux-gnu` for Linux 32 bit
+- `x86_64-pc-linux-gnu` for x86 Linux
 - `x86_64-w64-mingw32` for Win64
 - `x86_64-apple-darwin14` for macOS
 - `arm-linux-gnueabihf` for Linux ARM 32 bit

depends/packages.md

@@ -181,3 +181,18 @@ For us, it's much easier to just link a static `libsecondary` into a shared
 static or dynamic `libseconday`, that's not our concern. With a static
 `libseconday`, when we need to link `libprimary` into our executable, there's no
 dependency chain to worry about as `libprimary` has all the symbols.
+
+## Build targets:
+
+To build an individual package (useful for debugging), following build targets are available.
+
+    make ${package}
+    make ${package}_fetched
+    make ${package}_extracted
+    make ${package}_preprocessed
+    make ${package}_configured
+    make ${package}_built
+    make ${package}_staged
+    make ${package}_postprocessed
+    make ${package}_cached
+    make ${package}_cached_checksum