Ensure tianocore boot menu is shown for aarch64 workaround

[okurz]

#1275 removed the boot
menu by default to save 5s of boot time when the menu is not really
required. With the variable BOOT_MENU we can always request the menu
when it is necessary. On aarch64 to boot a non-default disk the
tianocore menu is required so for convience we can propose a different
default value to show the boot menu unless requested differently.

Related progress issue: https://progress.opensuse.org/issues/61910

[codecov]

Codecov Report

Merging #1334 into master will decrease coverage by 3.77%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1334      +/-   ##
==========================================
- Coverage   56.47%   52.69%   -3.78%     
==========================================
  Files          52       52              
  Lines        6130     6131       +1     
  Branches     1060     1061       +1     
==========================================
- Hits         3462     3231     -231     
- Misses       2255     2512     +257     
+ Partials      413      388      -25

Impacted Files	Coverage Δ
backend/qemu.pm	34.74% <0%> (-1.42%)	⬇️
backend/baseclass.pm	45.81% <0%> (-21.3%)	⬇️
consoles/vnc_base.pm	28.28% <0%> (-16.17%)	⬇️
consoles/sshIucvconn.pm	20% <0%> (-16%)	⬇️
consoles/VNC.pm	33.69% <0%> (-9.57%)	⬇️
consoles/network_console.pm	83.33% <0%> (-8.34%)	⬇️
consoles/console.pm	71.73% <0%> (-4.35%)	⬇️
OpenQA/Qemu/Proc.pm	66.83% <0%> (-3.07%)	⬇️
needle.pm	72.82% <0%> (-1.64%)	⬇️
... and 2 more

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