package/pkg-cargo: allow building in a sub-directory

Some packages have their rust sources as a sub-directory, rather
than at the root of the source tree.

Do like we do for autotools-package, and use the package's _SRCDIR
rather than the top-level directory $(@D).

Additionally, in such a situation, it is more than probable that
the Cargo.toml is also present in that sub-directory, so use that
when vendoring the package.

Signed-off-by: Yann E. MORIN <yann.morin@orange.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/adding-packages-cargo.txt

@@ -73,6 +73,11 @@ A few additional variables, specific to the Cargo infrastructure, can
 also be defined. Many of them are only useful in very specific cases,
 typical packages will therefore only use a few of them.
 
+* +FOO_SUBDIR+ may contain the name of a subdirectory inside the package
+  that contains the Cargo.toml file. This is useful, if for example, it
+  is not at the root of the tree extracted by the tarball. If
+  +HOST_FOO_SUBDIR+ is not specified, it defaults to +FOO_SUBDIR+.
+
 * +FOO_CARGO_ENV+ can be used to pass additional variables in the
   environment of +cargo+ invocations. It used at both build and
   installation time