[openwrt/openwrt] kernel-defaults: fix external kernel build when user_headers is missing

[LEDE Commits]

hauke pushed a commit to openwrt/openwrt.git, branch openwrt-21.02:
https://git.openwrt.org/0b0e97853a1ba4544185afb030aad81b38321c56

commit 0b0e97853a1ba4544185afb030aad81b38321c56
Author: Matt Merhar <mattmerhar at protonmail.com>
AuthorDate: Thu Feb 25 23:28:53 2021 -0500

    kernel-defaults: fix external kernel build when user_headers is missing
    
    Use an 'if' so the absence of $(LINUX_DIR)/user_headers doesn't make the
    line evaluate to false and cause the build to fail.
    
    Signed-off-by: Matt Merhar <mattmerhar at protonmail.com>
    (cherry picked from commit cc76e34c10e57cb1a4f7971268a77aad2f24c8ca)
---
 include/kernel-defaults.mk | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/kernel-defaults.mk b/include/kernel-defaults.mk
index e5a0ba367b..3aa1baa761 100644
--- a/include/kernel-defaults.mk
+++ b/include/kernel-defaults.mk
@@ -43,7 +43,9 @@ else
 		rmdir $(LINUX_DIR); \
 	fi
 	ln -s $(CONFIG_EXTERNAL_KERNEL_TREE) $(LINUX_DIR)
-	$(_SINGLE) [ -d $(LINUX_DIR)/user_headers ] && rm -rf $(LINUX_DIR)/user_headers
+	if [ -d $(LINUX_DIR)/user_headers ]; then \
+		rm -rf $(LINUX_DIR)/user_headers; \
+	fi
   endef
 endif