managerd: Harmonize the default bootlimit to 3

On U-Boot platforms the default bootlimit is set in the bootloader itself with CONFIG_BOOTCOUNT_BOOTLIMIT=3 (see the apertis/imx6/0003-mx6sabrelite-enable-bootcount.patch and apertis/renesas/0003-renesas-enable-bootcount.patch patches), so let's default to three failed boots before rollback here to have a more consistent behavior across platforms.

This in particular affects the UEFI platforms (amd64).

requested review from @fdanis

mentioned in merge request tests/apertis-test-cases!387 (merged)

merged

mentioned in merge request !86 (merged)