toolchain-external: add support for musl C library

This commit adds support for external toolchains based on the musl C
library, as available from http://www.musl-libc.org.

Note that the pre-built musl toolchains available from
http://musl.codu.org/ are not working for the moment, since they lack
sysroot support. However, this problem has been reported to the
maintainer, who has already added sysroot support in his scripts at
https://bitbucket.org/GregorR/musl-cross, and therefore the next
version of the pre-built toolchains should work with Buildroot
out-of-the-box. In the mean time, the musl-cross script must be used
to build the toolchain.

[Peter: reword comment]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
diff --git a/toolchain/toolchain-external/Config.in b/toolchain/toolchain-external/Config.in
index c53577e..89c799e 100644
--- a/toolchain/toolchain-external/Config.in
+++ b/toolchain/toolchain-external/Config.in
@@ -860,6 +860,10 @@
 	bool
 	select BR2_TOOLCHAIN_USES_UCLIBC
 
+config BR2_TOOLCHAIN_EXTERNAL_MUSL
+	bool
+	select BR2_TOOLCHAIN_USES_MUSL
+
 if BR2_TOOLCHAIN_EXTERNAL_CUSTOM
 
 choice
@@ -882,6 +886,13 @@
 	  eglibc configured to exclude key features may cause build failures to
 	  some packages.
 
+config BR2_TOOLCHAIN_EXTERNAL_CUSTOM_MUSL
+	bool "musl"
+	select BR2_TOOLCHAIN_EXTERNAL_MUSL
+	help
+	  Select this option if your external toolchain uses the
+	  'musl' C library, available from http://www.musl-libc.org/.
+
 endchoice
 
 if BR2_TOOLCHAIN_EXTERNAL_CUSTOM_UCLIBC
@@ -952,6 +963,7 @@
 config BR2_TOOLCHAIN_EXTERNAL_INET_RPC
 	bool "Toolchain has RPC support?"
 	select BR2_TOOLCHAIN_HAS_NATIVE_RPC
+	depends on !BR2_TOOLCHAIN_EXTERNAL_MUSL
 	default y if BR2_TOOLCHAIN_EXTERNAL_GLIBC
 	help
 	  Select this option if your external toolchain supports