package/pkg-meson: <pkg>_SUBDIR cleanup

No functional change is brought by this modification.

This patch:
* removes a redundant <pkg>_SRCDIR declaration (already defined in
  pkg-generic.mk)
* documents the usage of <pkg>_SUBDIR in the meson-specific section of
  the manual.

Signed-off-by: Lionel Orry <lionel.orry@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

docs/manual/adding-packages-meson.txt

@@ -85,6 +85,12 @@ A few additional variables, specific to the Meson 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 main meson.build file. This is useful,
+  if for example, the main meson.build file 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_CONF_ENV+, to specify additional environment variables to pass to
   +meson+ for the configuration step. By default, empty.
 

package/pkg-meson.mk

@@ -48,7 +48,6 @@ define inner-meson-package
 $(2)_CONF_ENV		?=
 $(2)_CONF_OPTS		?=
 $(2)_NINJA_ENV		?=
-$(2)_SRCDIR		= $$($(2)_DIR)/$$($(2)_SUBDIR)
 
 #
 # Configure step. Only define it if not already defined by the package