external: Utilize DESTDIR in shared makefile
external: Utilize DESTDIR in shared makefile

Signed-off-by: Patrick Williams <cbb7353e6d953ef360baf960c122346276c6e320@stwcx.xyz>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

shenki/skiboot,open-power/skiboot,stewart-ibm/skiboot,shenki/skiboot,shenki/skiboot,legoater/skiboot,open-power/skiboot,shenki/skiboot,stewart-ibm/skiboot,legoater/skiboot,open-power/skiboot,legoater/skiboot,stewart-ibm/skiboot,qemu/skiboot,shenki/skiboot,mikey/skiboot,qemu/skiboot,mikey/skiboot,legoater/skiboot,qemu/skiboot,mikey/skiboot,qemu/skiboot,open-power/skiboot,open-power/skiboot,legoater/skiboot,qemu/skiboot