buildman: Put the toolchain path first instead of last in PATH

If:

1. Toolchains A and B have the same filename
2. Toolchain A is in the PATH
3. Toolchain B is given in ~/.buildman and buildman uses it to build

then buildman will add toolchain B to the end of its path but will not
necessarily use it since U-Boot will find toolchain A first in the PATH.

Try to fix this by putting the toolchain first in the path instead of
last.
Signed-off-by: NSimon Glass <sjg@chromium.org>

tools/buildman/toolchain.py

@@ -89,7 +89,7 @@ class Toolchain:
         """
         env = dict(os.environ)
         env['CROSS_COMPILE'] = self.cross
-        env['PATH'] += (':' + self.path)
+        env['PATH'] = self.path + ':' + env['PATH']
         return env