linux: document and default to sensible value for BR2_LINUX_KERNEL_CUSTOM_GIT_VERSION

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

linux/Config.in

@@ -69,7 +69,11 @@ config BR2_LINUX_KERNEL_CUSTOM_GIT_REPO_URL
 
 config BR2_LINUX_KERNEL_CUSTOM_GIT_VERSION
 	string "Custom Git version"
+	default "HEAD"
 	depends on BR2_LINUX_KERNEL_CUSTOM_GIT
+	help
+	  Git revision to use in the format used by git rev-parse,
+	  E.G. a sha id, a tag, branch, ..
 
 config BR2_LINUX_KERNEL_VERSION
 	string