prereq-build: rename the openssl check to libssl, add back the old check - it is required after all

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42999 3c298f89-4303-0410-b956-a3cf2f4a3e73