configure bootloaders for kernel cpu microcode loading (MR 53)

The Linux kernel supports live patching CPU microcode on boot, something
we should definitely support. This patch adds support to grub and
sd-boot / bootloader spec config.

A new function is added to list out ucode in a directory (currently just
the output dir, which is almost always /boot). It's very basic at the
moment, it makes some pretty big assumptions about the name of the ucode
patch file (i.e. currently expects it to match *ucode.img) This if fine
for the ucode fw filenames that Alpine Linux / pmOS support. These
things can/should be improved later if we need to support more
situations.

For context/background, see: https://gitlab.com/postmarketOS/pmaports/-/issues/2370

Earlier versions of this patch tried to preserve writing the whole
configs for each bootloader in one call to `cat`, e.g. by generating the
list of lines to add ucode files, but it was a bit unruly (extraneous
newlines and formatting was inconsistent)