fastboot: getvar: correct/rename "has_slot" to "has-slot"

fastboot: getvar: correct/rename "has_slot" to "has-slot"

Since its inception in upstream fastboot android-n-preview-1 [1],
"has-slot" option has never taken the form of "has_slot". Amongst the
users of "getvar has-slot:" is the upstream bootloadertest.py [2].

Current U-Boot "has_slot" version must be a typo. Fix it.

[1] https://android.googlesource.com/platform/system/core/+/a797479bd51c
    ("Fix fastboot variable name")
[2] https://android.googlesource.com/platform/system/extras/+/72de393e118e3
    ("Bootloader verification for AndroidThings.")

Fixes: bab20504988f ("net: fastboot: Merge AOSP UDP fastboot")
Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Acked-by: Alex Kiernan <alex.kiernan@gmail.com>