fastboot: Remove "bootloader-version" variable

As per [1], there is no such fastboot variable as "bootloader-version".
Only "version-bootloader" is supported. Let's reflect this and not
confuse users further.

[1] https://android.googlesource.com/platform/system/core/+/refs/tags/android-q-preview-4/fastboot/README.md



Fixes: 3aab70af ("usb/gadget: add the fastboot gadget")
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>