rpi-cmdline: do_compile: Use pure Python syntax to get `CMDLINE`

[lluiscampos]

Otherwise the shell snippet fails with bad syntax when CMDLINE contains special characters like ${...} (useful to insert a U-Boot variable in cmdline.txt).

- What I did

Modify do_compile so that special characters ${...} are accepted for CMDLINE.

- How I did it

Getting CMDLINE variable through Python with d.getVar()

[lluiscampos]

@agherzan Thanks for your review and approval. What is the process to port patches to stable branches? I would like to bring this one to kirkstone.

[agherzan]

@lluiscampos As soon as it lands in master, just create another MR cherry-picking it for kirkstone. It'd be the fastest way.