infra: replace BUILDROOT_CONFIG with BR2_CONFIG

To make the naming consistent (all user-visible options should be
prefixed with BR2_).

An entry is added to Makefile.legacy to warn users who have set
BUILDROOT_CONFIG but not BR2_CONFIG.

Still export BUILDROOT_CONFIG but pointing to some phony value, to
make sure that scripts that still use it fail in a predictable way.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Jérémy Rosen <jeremy.rosen@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/scripts/mkusers

@@ -39,7 +39,7 @@ GROUP="${TARGET_DIR}/etc/group"
 # with /etc/group, so any use of gshadow must be conditional.
 GSHADOW="${TARGET_DIR}/etc/gshadow"
 
-# We can't simply source ${BUILDROOT_CONFIG} as it may contains constructs
+# We can't simply source ${BR2_CONFIG} as it may contains constructs
 # such as:
 #    BR2_DEFCONFIG="$(CONFIG_DIR)/defconfig"
 # which when sourced from a shell script will eventually try to execute
@@ -48,7 +48,7 @@ GSHADOW="${TARGET_DIR}/etc/gshadow"
 # So, we have to scan that file instead. Sigh... :-(
 PASSWD_METHOD="$( sed -r -e '/^BR2_TARGET_GENERIC_PASSWD_METHOD="(.*)"$/!d;'    \
                          -e 's//\1/;'                                           \
-                         "${BUILDROOT_CONFIG}"                                  \
+                         "${BR2_CONFIG}"                                        \
                 )"
 
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