toolchain/uClibc: properly honor DEBUG and STRIP options

The default uClibc configurations strip the resulting binaries/libraries
and don't add debug symbols which is a problem when wanting to debug
issues.
Bind the buildroot BR2_STRIP_* option to uClibc .config DO_STRIP and
BR2_ENABLE_DEBUG to uClibc DO_DEBUG.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>