package-ipkg: Do not fail build without base-files

package-ipkg: Do not fail build without base-files

If the base-files package is not selected, we will fail executing the
very first postinst script:

make[3]: Leaving directory `/local/users/fainelli/openwrt/trunk'
cp -fpR
/local/users/fainelli/openwrt/trunk/build_dir/target-arm_xscale_musl-1.1.15_eabi/root-orion
/local/users/fainelli/openwrt/trunk/build_dir/target-arm_xscale_musl-1.1.15_eabi/root.orig-orion
./usr/lib/opkg/info/busybox.postinst: line 3:
/local/users/fainelli/openwrt/trunk/build_dir/target-arm_xscale_musl-1.1.15_eabi/root-orion/lib/functions.sh:
No such file or directory
./usr/lib/opkg/info/busybox.postinst: line 4: default_postinst: command
not found
postinst script ./usr/lib/opkg/info/busybox.postinst has failed with
exit code 127
make[2]: *** [package/install] Error 1

Check for the existence of lib/functions.sh, and if it does not exist,
just bail out gracefully.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>