avbtool: Add 'calculate_kernel_cmdline' command.

This new command be used to concatenate the kernel cmdline descriptors
the same way libavb does. This is useful in situations where you want
to launch qemu without using a bootloader.

By design command-line substitutions are not performed, so e.g.

 - $(ANDROID_SYSTEM_PARTUIID)
 - $(ANDROID_BOOT_PARTUIID)
 - $(ANDROID_VBMETA_PARTUIID)
 - $(ANDROID_VERITY_MODE)

will need to be substituted by the caller of this command. A future CL
could add support for this via 'calculate_kernel_cmdline' options.

Additionally, automatically calculated kernel cmdline options appended
by avb_slot_verify() including

 - androidboot.veritymode
 - androidboot.veritymode.managed
 - androidboot.vbmeta.invalidate_on_error
 - androidboot.vbmeta.device_state
 - androidboot.vbmeta.{hash_alg, size, digest}
 - androidboot.vbmeta.device
 - androidboot.vbmeta.avb_version

are not set either. A future CL could add support for this via
'calculate_kernel_cmdline' options.

Bug: 78113934
Test: New unit tests and all unit tests pass.
Change-Id: I6dad8cdb88c754c5fb44fc102139505243d6c4d3