make_chroot: explicitly set path when entering the chroot.

The host system's PATH may not be match the one required by the SDK.
When going through the enter_chroot script it gets reset because bash is
invoked as a login shell but this doesn't happen when using the plain
old chroot command.

Fixes https://github.com/coreos/scripts/pull/290
enter_chroot: set reasonable PATH when calling locale-gen
Merge pull request #291 from marineam/path

make_chroot: explicitly set path when entering the chroot.