If we run e.g.

`make MYARGS=foo;bar`

we will get a build failure. The reason for this is that we extract all user-supplied arguments and store them in $(USER_MAKE_VARS). This is then used when calling make recursively like `make <target> $(USER_MAKE_VARS)`. In this case, it expands to `make images MYARGS=foo;bar`, which the shell will parse as two different command lines, the second one as a call to `bar`, which will fail, and thus fail the entire make process.

We do some massaging of $(USER_MAKE_VARS) already, but apparently need to add more shell escaping to it.