Two fixes:

(1) We don't need a custom install_kernel. We can install without
symbols by adding INSTALL_NODEBUG (which likely should be
WITHOUT_KERNEL_SYMBOLS_FILE, or something shorter) to CONF_INSTALL
(2) for make buildenv stage, use NANO_MAKE_CONF_BUILD rather than the
non-existant NANO_MAKE_CONF.

MFC after: 7 days