docs/manual: document $(FOO_PKGDIR)

In the manual, add a note that packages can use $(FOO_PKGDIR) to get the
path to the directory containg the .mk and Config.in files, if they need
it to e.g. copy files from there, like startup scripts, systemd units or
any other resource.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/adding-packages-generic.txt

@@ -495,6 +495,11 @@ endef
 
 In the action definitions, you can use the following variables:
 
+* +$(FOO_PKGDIR)+ contains the path to the directory containing the
+  +foo.mk+ and +Config.in+ files. This variable is useful when it is
+  necessary to install a file bundled in Buildroot, like a runtime
+  configuration file, a splashscren image...
+
 * +$(@D)+, which contains the directory in which the package source
   code has been uncompressed.