Add ST platfrom F429ZI to detected devices' pool

We need to support proper search when using mbedls detection:
```
yotta --plain search -k mbed-target:discovery-f429zi target
stm32f429i-disco-gcc 0.0.15:
    Official mbed build target for the mbed stm32f429i-disco development board.
    mbed-target:discovery-f429zi, mbed-official, stm32f429i-disco, gcc

additional results from https://yotta-private.herokuapp.com:
```

Author: PrzemekWirkus

mbed_lstools/lstools_base.py

@@ -86,7 +86,7 @@ def __init__(self):
         "0780": "ST_PLACEHOLDER",
         "0785": "ST_PLACEHOLDER",
         "0790": "ST_PLACEHOLDER",
-        "0795": "ST_PLACEHOLDER",
+        "0795": "DISCOVERY-F429ZI",
         "0799": "ST_PLACEHOLDER",
         "0805": "DISCO_L053C8",
         "0810": "DISCO_F334C8",