Recently we are introducing multiple git subtree projects and
it is the right time to have a universal script to update
various subtrees and replace the dts/update-dts-subtree.sh.

update-subtree.sh is a wrapper of git subtree commands.

Usage: From U-Boot top directory,
run
$ ./tools/update-subtree.sh pull <subtree-name> <release-tag>
for pulling a tag from the upstream.
Or run
$ ./tools/update-subtree.sh pick <subtree-name> <commit-id>
for cherry-pick a commit from the upstream.

Currently <subtree-name> supports dts, mbedtls and lwip.

Signed-off-by: Raymond Mao <raymond.mao@linaro.org>