Support different values in deviceinfo for upstream and downstream kernels

Created by: ollieparanoid

PR #1363 will make it possible to choose between upstream (official kernel.org ones, possibly slightly patched) and downstream kernels (the Android kernel forks) for devices.

As @drebrez and @z3ntu pointed out, we will need different values for the deviceinfo variables for upstream kernels.

@drebrez proposed the following solution which I quite like:

[...] having both lines in deviceinfo, e.g.:

deviceinfo_kernel_cmdline_mainline="cma=64m msm.vram=16m"
deviceinfo_kernel_cmdline_downstream="console=ttyHSL0,115200,n8"

but in the subpackage installation just remove all the other lines and rename the variable to deviceinfo_kernel_cmdline? That way we can have a common logic between all the APKBUILDs

I would recommend to name the suffix upstream instead of mainline though, because we have linux-postmarketos-mainline, linux-postmarketos-stable etc. We could implement the logic to create the kernel-specific deviceinfo file in the devicepkg-dev package we already have and use.