test-pkg: test a subset of toolchains by default, add -a and -n options

test-pkg: test a subset of toolchains by default, add -a and -n options

During the latest Buildroot Developers meeting, we discussed that
test-pkg would perhaps be more widely used if it tested a smaller
subset of toolchains. Indeed, it currently tests 47 toolchains, which
takes very long to build. Several of the toolchain configurations are
quite similar, and it is perhaps not necessary for contributors to
test them all before submitting a package.

Therefore, this commit changes the test-pkg script to only test a
subset of the toolchain configurations by default. The N first
configurations of the CSV files are tested, where N is hard-coded in
the script. The CSV file has therefore been re-organized to have the
first N toolchains be the most important ones.

A -a/--all option is added to test with all toolchains, while a
-n/--number option is added to test with the first N toolchains, N
being passed on the command line.

Note that the list of toolchains (built in the "toolchains" shell
variable) is no longer sorted. Indeed, when the first N toolchains are
tested, we want them to be tested in the same order as they are listed
in the CSV file, as we are careful to order them in an interesting
order. We only sort when all toolchains are tested.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Matt Weber <matthew.weber@rockwellcollins.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>