[build] Add build knowledge of dynamic libraries

Bonnibel will now build dynamic libraries when they're dependencies for
non-statically linked modules. It will also copy those shared libraries
into the initrd image for programs being copied into the image.

external/zstd.module

@@ -5,7 +5,6 @@ zstd = module("zstd",
     kind = "lib",
     copy_headers = True,
     deps = [ "libc" ],
-    output = "libzstd.a",
     sources = [
         "decompress/zstd_decompress.c",
         "decompress/zstd_ddict.c",