Merge pull request #2025 from pan-/nrf51_nano

Use nano lib by default for nrf51 based targets.

Author: 0xc0170

hal/targets.json

@@ -1092,7 +1092,8 @@
             "function": "MCU_NRF51Code.binary_hook",
             "toolchains": ["ARM_STD", "GCC_ARM"]
         },
-        "program_cycle_s": 6
+        "program_cycle_s": 6,
+        "default_build": "small"
     },
     "MCU_NRF51_16K_BASE": {
         "inherits": ["MCU_NRF51"],