Skip enabling BTF on kernel older than 5.10

BTF_KIND_VAR and BTF_KIND_DATASEC were introduced in 5.2.
It should be possible to enable BTF on 4.18 or above since libbpf can
load BTF.
Also, current code should work on 5.4 but this is untested.
For now, this CL enables BTF on 5.10 or above.

Test: boot on 4.9, 4.19, 5.10
Test: adb logcat | grep NetBpfLoad
Change-Id: I3da4970464afcf9a76b1f2645ed79c44750bcd6a
1 file changed