[source] build: remove old kernel-headers build directories

[LEDE Commits]

nbd pushed a commit to source.git, branch master:
https://git.lede-project.org/76b62e602233d1a2e1f8ef1c0833bd22ebaaf832

commit 76b62e602233d1a2e1f8ef1c0833bd22ebaaf832
Author: Felix Fietkau <nbd at nbd.name>
AuthorDate: Fri Jun 16 15:15:37 2017 +0200

    build: remove old kernel-headers build directories
    
    Saves space after updating kernel versions
    
    Signed-off-by: Felix Fietkau <nbd at nbd.name>
---
 toolchain/kernel-headers/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/toolchain/kernel-headers/Makefile b/toolchain/kernel-headers/Makefile
index bce6e93..1bf5598 100644
--- a/toolchain/kernel-headers/Makefile
+++ b/toolchain/kernel-headers/Makefile
@@ -71,6 +71,7 @@ define Host/Configure/post/mipsel
 endef
 
 define Host/Prepare
+	rm -rf $(BUILD_DIR_TOOLCHAIN)/linux-*
 	$(call Kernel/Prepare/Default)
 	rm -f $(BUILD_DIR_TOOLCHAIN)/linux
 	ln -s linux-$(LINUX_VERSION) $(BUILD_DIR_TOOLCHAIN)/linux