core/printvars: allow dumping a set of variables

core/printvars: allow dumping a set of variables

Dumping our 176164 variables can take quite some time (~12s here). What
takes the most time is sorting the variables (~9s), followed by the
parsing of our Makefiles (~3s), with the actual printing in the noise.

However, sometimes only one or a few variables are needed. For example,
one may want to retrieve the Linux build dir from a post-build hook (to
get the Linux' actual .config after our fixups and check for various
features).

Add the possibility to only dump the variables listed in $(VAR) which
must be passed as a make argument, like so:

    $ make -s printvars VARS="LINUX_DIR TOPDIR O"
    LINUX_DIR=/home/ymorin/dev/buildroot/O/build/linux-4.3 ($(BUILD_DIR)/$(LINUX_BASE_NAME))
    O=/home/ymorin/dev/buildroot/O/. (/home/ymorin/dev/buildroot/O/.)
    TOPDIR=/home/ymorin/dev/buildroot/buildroot (/home/ymorin/dev/buildroot/buildroot)

It is also possible to use make-appterns, like:

    $ make -s printvars VARS="BUSYBOX_%"

This is much faster (the time is just about the time it takes to parse
our Makefiles, 3s here) and easier to parse.

[Thomas: improve comment above the printvars target.]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>