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 @@ def get_all_active_memories(self, memory_list):
  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 @@ def _add_defines_from_region(
  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 @@ def add_regions(self):
  "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