common.sh: fix slot suffix detection for devices with /proc/bootconfig

root@ubuntu-phablet:/home/phablet# cat /proc/bootconfig 
androidboot.hardware = "mt6983"
androidboot.boot_devices = "bootdevice", "soc/112b0000.ufshci", "112b0000.ufshci"
androidboot.verifiedbootstate = "orange"
androidboot.force_normal_boot = "1"
androidboot.atm = "disabled"
androidboot.slot_suffix = "_a"
...

Hope they don't change formatting between devices/kernel versions.