include: Clean-up kernel git command options management

include: Clean-up kernel git command options management

The way git options are managed in kernel-defaults.mk makes additions

difficult. If requires different code path for each option; it's
ok so far as we handle only one option, but if we want to make the git
clone mechanism more flexible, more option will be required, which
will become tedious.

So; we'll move the GIT options into a variable, that may or may not be
set depending on the configuration, and we'll pass this variable to the
git command.

Signed-off-by: Mathieu Olivari <mathieu@qca.qualcomm.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40945 3c298f89-4303-0410-b956-a3cf2f4a3e73