mk: use linux and freebsd in config names

mk: use linux and freebsd in config names

Rather than using linuxapp and bsdapp everywhere, we can change things to
use the, more readable, terms "linux" and "freebsd" in our build configs.
Rather than renaming the configs we can just duplicate the existing ones
with the new names using symlinks, and use the new names exclusively
internally. ["make showconfigs" also only shows the new names to keep the
list short] The result is that backward compatibility is kept fully but any
new builds or development can be done using the newer names, i.e.  both
"make config T=x86_64-native-linuxapp-gcc" and "T=x86_64-native-linux-gcc"
work.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>