rename: @btf.o -> @mainline.o

(in prep for these actually being only loaded via mainline netbpfload)

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I9cea40b3d7525aa3ef8fdaad291b27d7d179a1aa
diff --git a/bpf_progs/offload.c b/bpf_progs/offload.c
index 90f96a1..dd59dca 100644
--- a/bpf_progs/offload.c
+++ b/bpf_progs/offload.c
@@ -24,16 +24,16 @@
 #define __kernel_udphdr udphdr
 #include <linux/udp.h>
 
-#ifdef BTF
+#ifdef MAINLINE
 // BTF is incompatible with bpfloaders < v0.10, hence for S (v0.2) we must
 // ship a different file than for later versions, but we need bpfloader v0.25+
 // for obj@ver.o support
 #define BPFLOADER_MIN_VER BPFLOADER_OBJ_AT_VER_VERSION
-#else /* BTF */
+#else /* MAINLINE */
 // The resulting .o needs to load on the Android S bpfloader
 #define BPFLOADER_MIN_VER BPFLOADER_S_VERSION
 #define BPFLOADER_MAX_VER BPFLOADER_OBJ_AT_VER_VERSION
-#endif /* BTF */
+#endif /* MAINLINE */
 
 // Warning: values other than AID_ROOT don't work for map uid on BpfLoader < v0.21
 #define TETHERING_UID AID_ROOT