Use a prebuilt grub image for EFI, creating BOOT/EFI/grub.cfg

There's a race condition (TOCTOU) between losetup --find
and applying the obtained path after additional dd in between;
twice awful as *at least* running both losetups closely would
reduce the window significantly, and reading the manpage
properly back then would eliminate this.
Reported-by: Anton Midyukov <antohami@altlinux.org>
Suggested-by: Gleb F-Malinovskiy <glebfm@altlinux.org>

This space is reserved for installation U-Boot.

Parameter "legacyboot" causes an error:
parted: invalid token: legacyboot

Booting from extlinux.conf is preferred over booting in EFI mode.

This space can be used by single board for u-boot.

The missing glob resulted in disk images properly
partitioned only for e2kv3 ("e2k" for historical
reasons in ALT), and v4+ ones only got a single
ext4 partition that wouldn't work with current
MCST's Boot code.

os-probe finds loopback devices.

While this argument can only have one "EFI" value.
Thus, the problem of building an image for Raspberry Pi on armh,
where there is no grub-efi, was solved.

extlinux.conf is not tied to the u-boot bootloader and always
needed replace Label with the UUID if the file exist.

Since the main goal for aarch64 and armh for us is Raspberry Pi,
it is always worth creating a fat32 partition for these arches.
At least for now.

For Raspberry Pi needed to build grub-efi + MBR images.

tar2fs does not run in hasher and without qemu. This causes build
problems for an architecture other than the host.

If build is performed on a system with EFI Secure Boot enabled,
then grub will not boot even on EFI without Secure Boot.
This problem is caused by the use of the --removable option, which
is not compatible with option --uefi-secure-boot  the current grub.
Option --uefi-secure-boot is enabled automatically if the system is
booted in EFI Secure Boot mode.

This commit fixes duplication of root device entry in /etc/fstab
and changes LABEL=ROOT entry with the proper one if found.

Apparently this didn't hurt qemu images much though...

This allows one not to hope that only the packages of the desired
loader will be installed.

Should be done this way in the first place it seems.

NB: 07-kernel change breaks multi-kernel setup!

Breaks: 650e92bf

Currently, only e2k requires specific disk partitioning.

INITRD_MODULES is reset further down the script.

Fixes: be75d3c3

Changing ROOTFS owner causes a problem:
created image's root directory is owned
by user executing make instead of root.

Changing WORKDIR owner is unnecessary
because it will be removed anyways.

The original commit broke system tar2fs use by accidentally
moving TOPDIR definition into a separate shell execution;
thanks iv@ for spotting and fixing it promptly.
Co-authored-by: Ivan A. Melnikov" <iv@altlinux.org>

...for vm images; BIOS one tested, EFI one untested yet
but will be required for AArch64 VMs it seems.

No sense to "provide" choice when there's none.

...so that umounts can happen reliably.
Reported-by: Gleb Fotengauer-Malinovskiy <glebfm@altlinux.org>

This one supports multiple kernels (and tweaks boot
labels aiming to simplify target kernel selection
for the CPU at hand) instead of an earlier attempt
to fill in a template.

No need for the template so just drop it.

This was crucial for e2k rescue image supporting
multiple CPU versions but can be useful elsewhere.

The current state made vm images belong to root group,
no reason to not change those to the primary group of
the user building an image.

kpartx -d -s could fail in some circumstances,
make a safety cleanup call more verbose.

Somewhat ARM-like with only the bootloader bits
standing out; Elbrus 2000 firmware can read ext2
and boot the kernel directly.