core: enhance printvars

core: enhance printvars

Currently, the output of printvars copntains the name of the variable,
its expanded value and its un-expanded value.

However, most of the time, we need the actual, expanded value, so it can
be re-used from a (non-Buildroot) infrastructure script, like a
post-build script, or a build-farm driver (e.g. a Jenkins job...)

Add two options that a user may set to change the output of printvars:
  - QUOTED_VARS, if set, will quote the value
  - RAW_VARS, if set, will print the unexpanded value

The new output by default only prints the expanded value now.

So that it can be used as such:

    $ make -s printvars VARS=BUSYBOX_VERSION
    BUSYBOX_VERSION=1.26.2

    $ make -s printvars VARS=BUSYBOX_RDEPENDENCIES QUOTED_VARS=YES
    BUSYBOX_RDEPENDENCIES='ncurses util-linux'

    $ make -s printvars VARS=BUSYBOX_FINAL_PATCH_DEPENDENCIES RAW_VARS=YES
    BUSYBOX_FINAL_PATCH_DEPENDENCIES=$(sort $(BUSYBOX_PATCH_DEPENDENCIES))

And it is even possible to directly evaluate it in a shell script:

    eval $(make -s printvars VARS=BUSYBOX_VERSION QUOTED_VARS=YES)

Backward compatibility of the output is not maintained. It is believed
that scripts that depended on the previous output were very fragile to
begin with, because they had to filter the non-formatted output
(splitting on spaces or braces was not really possible, because values
could contain either).

Document printvars and its options in the manual; list it in the output
of 'make help'.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>