support/scripts/mkgrubimg: Change command-line option to `-c`

support/scripts/mkgrubimg: Change command-line option to `-c`

Change command-line option to `-c` to make it more intuitive.

Furthermore, make it so that the actual received argument is a file
containing the command-line.

Signed-off-by: Sergiu Moga <sergiu.moga@protonmail.com>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #910