Upgrade OUTPUT_EXT and use it to pick binary type

targets.json contained a key for some targets, `OUTPUT_EXT`, which I
moved to `Target`, the root of all targets. Following on that, the tools
now use this extension provided by `OUTPUT_EXT` to determine the file
type of the output executable.

tools/toolchains/__init__.py

@@ -1003,7 +1003,7 @@ class mbedToolchain:
 
         filename = name+'.'+ext
         elf = join(tmp_path, name + '.elf')
-        bin = join(tmp_path, filename)
+        bin = None if ext is 'elf' else join(tmp_path, filename)
         map = join(tmp_path, name + '.map')
 
         r.objects = sorted(set(r.objects))
@@ -1012,7 +1012,7 @@ class mbedToolchain:
             self.progress("link", name)
             self.link(elf, r.objects, r.libraries, r.lib_dirs, r.linker_script)
 
-        if self.need_update(bin, [elf]):
+        if bin and self.need_update(bin, [elf]):
             needed_update = True
             self.progress("elf2bin", name)
             self.binary(r, elf, bin)

tools/toolchains/arm.py

@@ -216,8 +216,10 @@ class ARM(mbedToolchain):
 
     @hook_tool
     def binary(self, resources, elf, bin):
+        _, fmt = splitext(bin)
+        bin_arg = {".bin": "--bin", ".hex": "--i32"}[fmt]
         # Build binary command
-        cmd = [self.elf2bin, '--bin', '-o', bin, elf]
+        cmd = [self.elf2bin, bin_arg, '-o', bin, elf]
 
         # Call cmdline hook
         cmd = self.hook.get_cmdline_binary(cmd)

tools/toolchains/gcc.py

@@ -261,7 +261,9 @@ class GCC(mbedToolchain):
     @hook_tool
     def binary(self, resources, elf, bin):
         # Build binary command
-        cmd = [self.elf2bin, "-O", "binary", elf, bin]
+        _, fmt = splitext(bin)
+        bin_arg = {'.bin': 'binary', '.hex': 'ihex'}[fmt]
+        cmd = [self.elf2bin, "-O", bin_arg, elf, bin]
 
         # Call cmdline hook
         cmd = self.hook.get_cmdline_binary(cmd)

tools/toolchains/iar.py

@@ -224,8 +224,10 @@ class IAR(mbedToolchain):
 
     @hook_tool
     def binary(self, resources, elf, bin):
+        _, fmt = splitext(bin)
+        bin_arg = {".bin": "--bin", ".hex": "--ihex"}[fmt]
         # Build binary command
-        cmd = [self.elf2bin, "--bin", elf, bin]
+        cmd = [self.elf2bin, bin_arg, elf, bin]
 
         # Call cmdline hook
         cmd = self.hook.get_cmdline_binary(cmd)