bpf-headers: add a package with kernel headers for ebpf

In order to genererate suitable kernel headers, a 5.10 kernel tree is
prepared with a default config for mips. The arch is forced to mips in
order to avoid issues with inline asm on various architectures in a way
that doesn't involve relying on the host toolchain/headers.
It also has the advantage of supporting both endian types

Signed-off-by: Felix Fietkau <nbd@nbd.name>

package/kernel/bpf-headers/src/include/generated/bounds.h

@@ -0,0 +1,14 @@
+#ifndef __LINUX_BOUNDS_H__
+#define __LINUX_BOUNDS_H__
+/*
+ * DO NOT MODIFY.
+ *
+ * This file was generated by Kbuild
+ */
+
+#define NR_PAGEFLAGS 23 /* __NR_PAGEFLAGS */
+#define MAX_NR_ZONES 4 /* __MAX_NR_ZONES */
+#define NR_CPUS_BITS 1 /* ilog2(CONFIG_NR_CPUS) */
+#define SPINLOCK_SIZE 64 /* sizeof(spinlock_t) */
+
+#endif