Fix MBED_RAM_START/MBED_RAM_SIZE symbol generation

1. Fix MBED_RAM_START/MBED_RAM_SIZE are not generated when there are
   target.mbed_ram_start/target.mbed_ram_size overrides
2. Fix MBED_RAM_START/MBED_RAM_SIZE are duplicated.

tools/config/__init__.py

@@ -768,8 +768,6 @@ class Config(object):
                         mem_size = size
                     memory = 'ROM'
                 elif memory in ['IRAM1', 'SRAM_OC', 'SRAM_UPPER', 'SRAM']:
-                    if (self.has_ram_regions):
-                        continue
                     start, size = self._get_primary_memory_override("ram")
                     if start:
                         mem_start = start

tools/toolchains/mbed_toolchain.py

@@ -840,7 +840,7 @@ class mbedToolchain:
     def _add_all_regions(self, region_list, active_region_name):
         for region in region_list:
             self._add_defines_from_region(region)
-            if region.active:
+            if region.active and active_region_name:
                 for define in [
                         ("%s_START" % active_region_name,
                          "0x%x" % region.start),
@@ -874,7 +874,7 @@ class mbedToolchain:
                     "s" if len(regions) > 1 else "",
                     ", ".join(r.name for r in regions)
                 ))
-                self._add_all_regions(regions, "MBED_RAM")
+                self._add_all_regions(regions, None)
             except ConfigException:
                 pass