Update asterisk-debug.md: append libbfd as the dependency

This package is requred when enabling more debug options for Asterisk.

asterisk-debug.md

@@ -19,13 +19,25 @@ CONFIG_PACKAGE_gdb=y
 
 gdb will be installed and running on the target directly.
 
-### Enable package libbfd
-
-This package is requred when enabling more debug options for Asterisk. Run ```make menuconfig``` and enable ```Libraries → libbfd```
+### Append libbfd as the dependency
+
+This package is requred when enabling more debug options for Asterisk.
+
+```
+diff --git a/net/asterisk/Makefile b/net/asterisk/Makefile
+index 824b9e7..ab85f07 100644
+--- a/net/asterisk/Makefile
++++ b/net/asterisk/Makefile
+@@ -463,7 +463,7 @@ $(call Package/$(PKG_NAME)/Default)
+   TITLE:=Complete open source PBX, v$(PKG_VERSION)
+   MENU:=1
+   DEPENDS:=+libstdcpp +jansson +libcap +libedit +libopenssl +libsqlite3 +libuuid +libxml2 +zlib \
+-          +libncurses +libpopt +libxslt +ubus +libpicoevent
++          +libncurses +libpopt +libxslt +ubus +libpicoevent +libbfd
+ endef
+ 
+ define Package/$(PKG_NAME)/description
 
-```
-~/git/iopsyswrt$ grep libbfd .config
-CONFIG_PACKAGE_libbfd=y
 ```
 
 ### Build An Entire Firmware as Usual First