fastboot: getvar: Add "is-userspace" variable

As per documentation [1], Android-Q requires for bootloader to provide
"is-userspace" variable. "no" value should be always returned. This
patch implements this fastboot variable.

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



Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>