getvar: fix broken 'fastboot flash' when partition has slots

slots ourselves using a Fastboot command Checking Android's Currently Active

rc The bootloader successfully marked the slot as bootable Before first boot: $ fastboot getvar all slot-count

fastboot getvar currentslot $ fastboot getvar version -bootloader version -bootloader : 8996-012001 ▷ System runs from current slot - other slot is not used ▷ One slot is

fastboot change slot output from fastboot getvar all : security-patch-level: current-slot:b