kernel-install: support the case /etc/machine-id is missing or empty (#5975)

Some .install plugins does not require that machine ID is set such as
20-grubby.install for Fedora and 50-depmod.install.
To support such plugins to run without valid machine-id, this commit
makes the following change:
* if /etc/machine-id is missing or empty, create temporary directory
  and set its path to BOOT_DIR_ABS,
* run the .install helpers with KERNEL_INSTALL_MACHINE_ID environment
  variable that'd be empty if /etc/machine-id is missing or empty.
This may be useful for installing kernel for e.g. stateless systems
which initialize machine-id while booting the systems.