After you build successfull your own firmware you have a crosscompiler, which you can use to build own programs for the ADM5120 target.

Use the crosscompiler from your toolchain standalone

trunk/openwrt/staging_dir_mipsel/bin/mipsel-linux-uclibc-gcc -o hello hello.c

Because there is never enough space in the flash strip the resulting executable

trunk/openwrt/staging_dir_mipsel/mipsel-linux-uclibc/bin/strip hello

To have this program installed with your firmware, copy it into the tree

cp hello trunk/openwrt/target/linux/image/adm5120/base-files/sbin/

Read in trunk/openwrt/docs/buildroot-documentation.html in your tree or http://midge.vlad.org.ua/svn/trunk/openwrt-midge/docs/buildroot-documentation.html online.