toolchain: include C libraries in legal-info

So far, the legal-info infrastructure was not exporting the legal
info for the C libraries. This is because the legal-info only acts
on packages defined in $(TARGETS).

But the C libraries are never added to $(TARGETS), since there is no
corresponding BR2_PACKAGE_<C-LIBRARY>.

This patch adds such symbols for the 4 C libraries we support in our
internal backend: uClibc, glibc, eglibc and musl.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/glibc/Config.in

@@ -1,5 +1,10 @@
 if BR2_TOOLCHAIN_BUILDROOT_EGLIBC
 
+# For legal-info
+config BR2_PACKAGE_EGLIBC
+	bool
+	default y
+
 choice
 	prompt "eglibc version"
 	default BR2_EGLIBC_VERSION_2_18
@@ -18,6 +23,11 @@ endif
 
 if BR2_TOOLCHAIN_BUILDROOT_GLIBC
 
+# For legal-info
+config BR2_PACKAGE_GLIBC
+	bool
+	default y
+
 choice
 	prompt "glibc version"
 	default BR2_GLIBC_VERSION_2_18

package/musl/Config.in

@@ -0,0 +1,5 @@
+# For legal-info
+config BR2_PACKAGE_MUSL
+	bool
+	depends on BR2_TOOLCHAIN_USES_MUSL
+	default y

package/uclibc/Config.in

@@ -1,5 +1,10 @@
 if BR2_TOOLCHAIN_BUILDROOT_UCLIBC
 
+# For legal-info
+config BR2_PACKAGE_UCLIBC
+	bool
+	default y
+
 comment "uClibc Options"
 
 choice