[make] allow to use dynamically loaded plugins

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[devel] allow to build js engine plugins separately

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[rpm] do not use curl

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[rpm] js engine plugins devel package

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>