build: make git sub-modules to fetch configurable

build: make git sub-modules to fetch configurable

Currently the git protocol downloads all submodules of the target
repository. This can be unwieldy for repositories with a lot of submodules
where only a subset are required in the context of the OpenWrt build.

This change adds a PKG_SOURCE_SUBMODULES variable to configure this
behavior. It takes a space-separated list of submodule paths, or the word
"skip" to disable submodule downloads entirely. The default is to download
all submodules, i.e. preserving current behavior.

Signed-off-by: Karsten Sperling <ksperling@apple.com>