toolchain/kernel-headers: add 2.6.28

Don't make it default yet as uclibc/busybox seems to have some issues
with it.

toolchain/kernel-headers/Config.in

@@ -67,6 +67,9 @@ choice
 	config BR2_KERNEL_HEADERS_2_6_27
 		bool "Linux 2.6.27.x kernel headers"
 
+	config BR2_KERNEL_HEADERS_2_6_28
+		bool "Linux 2.6.28.x kernel headers"
+
 	config BR2_KERNEL_HEADERS_SNAP
 		bool "Linux 2.6 snapshot"
 
@@ -114,5 +117,6 @@ config BR2_DEFAULT_KERNEL_HEADERS
 	default "2.6.25.19"	if BR2_KERNEL_HEADERS_2_6_25
 	default "2.6.26.8"	if BR2_KERNEL_HEADERS_2_6_26
 	default "2.6.27.8"	if BR2_KERNEL_HEADERS_2_6_27
+	default "2.6.28"	if BR2_KERNEL_HEADERS_2_6_28
 	default "2.6"		if BR2_KERNEL_HEADERS_SNAP