uclibc: install headers after building headers in configure part

This patch will run "make headers install_headers" instead of the old "make
headers install_dev". The latter requires a cross compiler available in uClibc
versions > 0.9.29, and the cross compiler is not available at the configuration
time of uClibc.

Spotted and fixed by Bernhard Fischer.

Tested-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>