core: do not accept multiple definitions of a package

One of the selling points for br2-external is to provide a mean to add
new packages. However, it is not supported that a package be defined by
Buildroot and then redefined in a br2-external tree.

This situation may occur without the user noticing or even willing to
redefine the package, for example:
  - br2-external is first created against a version of Buildroot
  - a package (missing in Buildroot) is added to that br2-external tree
  - upstream Buildroot adds this package
  - user updates to the new Buildroot

In this case, the result in undefined, and we can't make any guarantee
on the result (working or not).

Add a sanity check so that a package redefinition gets caught.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Makefile

@@ -334,6 +334,7 @@ unexport O
 GNU_HOST_NAME := $(shell support/gnuconfig/config.guess)
 
 PACKAGES :=
+PACKAGES_ALL :=
 
 # silent mode requested?
 QUIET := $(if $(findstring s,$(filter-out --%,$(MAKEFLAGS))),-q)

package/pkg-generic.mk

@@ -341,6 +341,14 @@ endef
 
 define inner-generic-package
 
+# Ensure the package is only declared once, i.e. do not accept that a
+# package be re-defined by a br2-external tree
+ifneq ($(call strip,$(filter $(1),$(PACKAGES_ALL))),)
+$$(error Package '$(1)' defined a second time in '$(pkgdir)'; \
+	previous definition was in '$$($(2)_PKGDIR)')
+endif
+PACKAGES_ALL += $(1)
+
 # Define default values for various package-related variables, if not
 # already defined. For some variables (version, source, site and
 # subdir), if they are undefined, we try to see if a variable without
@@ -351,6 +359,7 @@ define inner-generic-package
 $(2)_TYPE                       =  $(4)
 $(2)_NAME			=  $(1)
 $(2)_RAWNAME			=  $$(patsubst host-%,%,$(1))
+$(2)_PKGDIR			=  $(pkgdir)
 
 # Keep the package version that may contain forward slashes in the _DL_VERSION
 # variable, then replace all forward slashes ('/') by underscores ('_') to