examples/natmod/btree: Fix build for Xtensa.

This commit provides the appropriate external symbol addresses to let
the "btree" example natmod build for the Xtensa platform.

On the ESP8266, unsigned integer division code isn't provided as part of
libgcc.a, libm.a, or libc.a, but it is instead provided by the ROM.
Regular builds inject the appropriate symbol addresses as part of the
linking process (see eagle.rom.addr.v6.ld), but natmods need this
information brought in from somewhere else.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

Author: agatti

examples/natmod/btree/Makefile

@@ -8,7 +8,7 @@ MOD = btree_$(ARCH)
 SRC = btree_c.c
 
 # Architecture to build for (x86, x64, armv7m, xtensa, xtensawin, rv32imc)
-ARCH = x64
+ARCH ?= x64
 
 BTREE_DIR = $(MPY_DIR)/lib/berkeley-db-1.xx
 BERKELEY_DB_CONFIG_FILE ?= \"extmod/berkeley-db/berkeley_db_config_port.h\"
@@ -32,6 +32,10 @@ SRC += $(addprefix $(realpath $(BTREE_DIR))/,\
 	mpool/mpool.c \
 	)
 
+ifeq ($(ARCH),xtensa)
+MPY_EXTERN_SYM_FILE=$(MPY_DIR)/ports/esp8266/boards/eagle.rom.addr.v6.ld
+endif
+
 include $(MPY_DIR)/py/dynruntime.mk
 
 # btree needs gnu99 defined