Update mbed_targets.md

Update device_name section with more recent content from mbed-os / docs

Author: Amanda Butler

docs/advanced/mbed_targets.md

@@ -167,9 +167,7 @@ The hook code can look quite different for different targets. Take a look at the
 
 ## device_name
 
-Passes necessary data for exporting the mbed code to various third party tools and IDEs.
+This property is used to pass necessary data for exporting the mbed code to various third party tools and IDEs.
 
-This is possible because the device name corresponds to a field in publicly hosted CMSIS packs. These packs hold target properties. [This](http://www.keil.com/pack/Keil.Kinetis_K20_DFP.pdsc) is the PDSC that contains the TEENSY_31 device (MK20DX256xxx7). The device information begins on line 156. The dname (device name) field on line 156 directly corresponds to that in the Uvision5 IDE target selection window. Beginning on line 15 of `tools/export/uvision/uvision.tmpl`, target information from these packs is used to generate valid Uvision5 projects. If the device name is not found, we use a generic ARM CPU target in Uvision5.
-
-`tools/export/iar/iar_definitions.json` uses this device name to store information necessary to set the target in an IAR project.
+Please see [exporters.md](exporters.md) for information about this field.