When you want to have a git source then you need to use git but you need to specify the protocol as well.
Format: git://URL;protocol=PROTOCOL;branch=BRANCH
Also you need to specify the revision with SRCREV
For https protocol the repo needs to be public, if it is private then you need to save your git credentials to the system's git configuration.
For ssh protocol then you need to set up your public ssh key in your github account.
So, for your project, it would look like this:
SRC_URI = "git://github.com/PROJECT/linux-imx;protocol=https;branch=BRANCH"
SRCREV = "aaaa.."
or
SRC_URI = "git://github.com/PROJECT/linux-imx;protocol=ssh;branch=BRANCH"
SRCREV = "aaaa.."
It remains to set the branch and SRCREV
SRCREV is the commit hash that you want to fetch.
Regarding the problem you faced lately which is Missing SRC_URI checksum is that if you set the SRC_URI protocol to https instead of git like:
SRC_URI = "https://github.com/PROJECT/linux-imx"
then, bitbake will assume that linux-imx is a regular file to download, then it will look for SRC_URI checksum to verify the integrity of the file which in your case don't needed.