I don't think this is the right way to fix CFLAGS/LDFLAGS. Instead swupdate should respect these flags.
Possibly something like(untested):
diff --git a/Makefile b/Makefile
index 3421b24b..7d9577a8 100644
--- a/Makefile
+++ b/Makefile
@@ -411,8 +411,8 @@ quiet_cmd_swupdate = LD $@
cmd_swupdate = $(srctree)/scripts/trylink \
"$@" \
"$(CC)" \
- "$(KBUILD_CFLAGS) $(CFLAGS_swupdate)" \
- "$(LDFLAGS) $(EXTRA_LDFLAGS) $(LDFLAGS_swupdate)" \
+ "$(KBUILD_CFLAGS) $(CFLAGS_swupdate) $(CFLAGS)" \
+ "$(EXTRA_LDFLAGS) $(LDFLAGS_swupdate) $(LDFLAGS)" \
"$(swupdate-objs) $(ipc-lib)" \
"$(swupdate-libs)" \
"$(LDLIBS)"
@@ -424,8 +424,8 @@ quiet_cmd_addon = LD $@
cmd_addon = $(srctree)/scripts/trylink \
"$@" \
"$(CC)" \
- "$(KBUILD_CFLAGS) $(CFLAGS_swupdate)" \
- "$(LDFLAGS) $(EXTRA_LDFLAGS) $(LDFLAGS_swupdate) -L$(objtree)" \
+ "$(KBUILD_CFLAGS) $(CFLAGS_swupdate) $(CFLAGS)" \
+ "$(EXTRA_LDFLAGS) $(LDFLAGS_swupdate) $(LDFLAGS) -L$(objtree)" \
"$(2)" \
"$(swupdate-libs)" \
"$(LDLIBS) :${swupdate-ipc-lib}"
@@ -435,8 +435,8 @@ quiet_cmd_shared = LD $@
"$@" \
"$(CC)" \
"-shared -Wl,-soname,$@" \
- "$(KBUILD_CFLAGS) $(CFLAGS_swupdate)" \
- "$(LDFLAGS) $(EXTRA_LDFLAGS) $(LDFLAGS_swupdate) -L$(objtree)" \
+ "$(KBUILD_CFLAGS) $(CFLAGS_swupdate) $(CFLAGS)" \
+ "$(EXTRA_LDFLAGS) $(LDFLAGS_swupdate) $(LDFLAGS) -L$(objtree)" \
"$(2)" \
"$(LDLIBS)"