Add EFM32 Pearl Gecko to the library builds in anticipation of release. (#1668)

Author: stevew817

workspace_tools/build_release.py

@@ -133,6 +133,7 @@
     ('EFM32LG_STK3600',     ('ARM', 'GCC_ARM', 'uARM')),
     ('EFM32GG_STK3700',     ('ARM', 'GCC_ARM', 'uARM')),
     ('EFM32WG_STK3800',     ('ARM', 'GCC_ARM', 'uARM')),
+    ('EFM32PG_STK3401',     ('ARM', 'GCC_ARM', 'uARM')),
 
     ('MAXWSNENV', ('ARM', 'GCC_ARM', 'IAR')),
     ('MAX32600MBED', ('ARM', 'GCC_ARM', 'IAR')),