managerd: Harmonize the default bootlimit to 3
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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).