packer_ubuntu.json: Rename packer_config.json

Rename packer_config.json to packer_ubuntu.json in order to
differentiate the config for FreeBSD and Ubuntu

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: I824ebe19865968af531ee327ce8ae40617680dcf
diff --git a/packer_config.json b/packer_ubuntu.json
similarity index 100%
rename from packer_config.json
rename to packer_ubuntu.json