fastboot: Fix slot names reported by getvar

fastboot: Fix slot names reported by getvar

In commit [1] fastboot tool was changed w.r.t. new A/B specification [2],
and now we should report slot names in "a" format instead of "_a".
Latter is now considered legacy and we shouldn't rely on that anymore.

Due to this one can observe next error with recent fastboot tool:

    $ fastboot flash boot boot.img
    Sending 'boot__a' (11301 KB)
        OKAY [  0.451s]
    Writing 'boot__a'
        FAILED (remote: 'cannot find partition')
    fastboot: error: Command failed

Let's use new slot format in order to fix double underscores "__" and to
be in sync with AOSP master.

[1] https://android.googlesource.com/platform/system/core/+/8091947847d5e5130b09d2ac0a4bdc900f3b77c5
[2] https://source.android.com/devices/tech/ota/ab/ab_implement#partitions

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>