split --prebuilt and --host into --gcc-which and --qemu-which

Only one --host exists at ./build-modules, since that can select the host
kernel, which is independent from the toolchain.

Document that user mode simulation stopped working.

build-baremetal

@@ -7,6 +7,9 @@ from shell_helpers import LF
 class Main(common.BuildCliFunction):
     def __init__(self):
         super().__init__(
+            defaults={
+                'gcc_which':'crosstool-ng',
+            },
             description='''\
 Build the baremetal examples with crosstool-NG.
 ''',
@@ -31,11 +34,7 @@ Build the baremetal examples with crosstool-NG.
             '-mcpu={}'.format(self.env['mcpu']), LF,
             '-nostartfiles', LF,
         ]
-        if self.env['prebuilt']:
-            gcc = 'arm-none-eabi-gcc'
-        else:
-            os.environ['PATH'] = self.env['crosstool_ng_bin_dir'] + os.environ['PATH']
-            gcc = self.get_toolchain_tool('gcc', allowed_toolchains=['crosstool-ng'])
+        gcc = self.get_toolchain_tool('gcc')
         if self.env['emulator'] == 'gem5':
             if self.env['machine'] == 'VExpress_GEM5_V1':
                 entry_address = 0x80000000