Assuming that your build directory is on the same level that opencv source,
From the build directory run
../opencv/ios/configure-device_xcode.sh
or
../opencv/ios/configure-simulator_xcode.sh
Then from the same folder invoke
xcodebuild -sdk iphoneos -configuration Release -target ALL_BUILD
xcodebuild -sdk iphoneos -configuration Release -target install install
or
xcodebuild -sdk iphonesimulator -configuration Release -target ALL_BUILD
xcodebuild -sdk iphonesimulator -configuration Release -target install install