implement ucode and uki hooks

[dkwo]

I received these patches from Daniel Semyonov.
I have not tested them yet, but I figured I would post them here so that anyone can take a look.

Some notes from the author:

• As tinyramfs uses POSIX sh, I made sure to only use POSIX features of
  other commands I called (awk, sed, grep, wc).
  For awk specifically, the code I wrote didn't work for gawk without
  the -Wposix flag, but testing with 3 other implementations (those
  available in Void) indicates they only emit a warning with this flag
  to stderr.
• As I used objcopy (which isn't really standardized AFAIK) to build the
  UKI, I added support for both GNU's objcopy (which also need objdump)
  and llvm-objcopy.
• The microcode hook adds only the current CPU's microcode in hostonly
  mode (it generates an image from the firmware files, though I only
  tested this hook on an AMD machine), unless a prebuilt image is
  specified (in which case it just uses it).

[dkwo]

My comments: After looking at the patches, they mostly look fine. This is quite close to the scripts I've been already using for a while, except for:

1. I was passing --set-section-flags to llvm-objcopy. I now tested without, and it works indeed fine.

2. At least for intel && hostonly, I was using iucode_tool -S --write-earlyfw=${tmpdir}/ucode /usr/lib/firmware/intel-ucode/* instead, and I wonder if this simpler command is preferred.

[dkwo]

At the moment, it seems that intel code uses decimal, but it should use hex.
Btw, I think exporting DM_DISABLE_UDEV=1 in cryptsetup hook is not needed, while it's enough to do it for lvm.