Rename WEBOS_DISTRO_BUILD_INFO to WEBOS_DISTRO_BUILD_ID

- An optionally prefixed build number is better described as a "build
  ID" than as "build info".
- Tweak the descriptions of the build-related variables.

[GF-2613] Invent build variable with build number and Jenkins job name

Open-webOS-DCO-1.0-Signed-off-by: Herb Kuta <herb.kuta@lge.com>
Change-Id: I8ebc2a9df42318095a920518303459b5467b0986
Reviewed-on: https://g2g.palm.com/1847
Reviewed-by: Build Verification
Tested-by: Herb Kuta <herb.kuta@lge.com>
Reviewed-by: Pekka Ropo <pekka.ropo@palm.com>
Reviewed-by: Keith Derrick <keith.derrick@lge.com>