Cleanup Makefile

- Add missing rootfs_clean rule
- Remove unused defines
- Rename some rules

This fix #6 (GitHub issue)

Signed-off-by: Pascal Brand <pascal.brand@st.com>
Fix tee-supplicant link error and parallel build error (make -jX).

- Move -lteec at the end of the link line to fix the following undefined
reference:

$ make CROSS_COMPILE=arm-linux-gnueabihf-
...
tee_supplicant.c:(.text+0xdc): undefined reference to `_dprintf'
...

- Add makefile dependency to fix the following:

$ make -j3 CROSS_COMPILE=arm-linux-gnueabihf-
...
cp out/libteec/libteec.so* out/export/lib
cp: cannot stat ‘out/libteec/libteec.so*’: No such file or directory
make: *** [copy_export] Error 1