esp8266/Makefile: Bump axTLS TLS record buffer size to 5K.

The latest fashion is pushing certificate sub-chains, instead of a single
certificate, during TLS handshake. These are pushed via single TLS record
and effectively put minimum size limit on TLS record buffer. Recently,
these commonly grew over 4K, so we have little choice but to adjust.

esp8266/Makefile

@@ -234,7 +234,7 @@ $(BUILD)/libaxtls.a:
 	cd ../lib/axtls; cp config/upyconfig config/.config
 	cd ../lib/axtls; $(MAKE) oldconfig -B
 	cd ../lib/axtls; $(MAKE) clean
-	cd ../lib/axtls; $(MAKE) all CC="$(CC)" LD="$(LD)" AR="$(AR)" CFLAGS_EXTRA="$(CFLAGS_XTENSA) -Dabort=abort_ -DRT_MAX_PLAIN_LENGTH=1024 -DRT_EXTRA=3072"
+	cd ../lib/axtls; $(MAKE) all CC="$(CC)" LD="$(LD)" AR="$(AR)" CFLAGS_EXTRA="$(CFLAGS_XTENSA) -Dabort=abort_ -DRT_MAX_PLAIN_LENGTH=1024 -DRT_EXTRA=4096"
 	cp ../lib/axtls/_stage/libaxtls.a $@
 
 clean-modules: