package/pkg-utils: show-info: report the package build directory

package/pkg-utils: show-info: report the package build directory

Some people want to be able to inspect the current state of the build,
and report information about it, like:
  - report the number of packages already built out of the total,
  - list the packages being actually built (e.g. for TLPB)
  - etc...

However, the location where a package is built is inherently an internal
detail, so expose that to the user in the output of show-info. We only
expose the location relative to the base directory (basically, either
output/ or the user-suppiled $(O)), so that show-info does not contain
local information (the output of show-info can be shared).

Interested parties will be able to poke in there to identify the stamp
files and deduce the package's state.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Vadim Kochan <vadim4j@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: eeppeliteloop@gmail.com
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>