support/scripts: prepare handling virtual packages in generated lists

support/scripts: prepare handling virtual packages in generated lists

Prepare to tell apart real packages from virtual packages.

Currently, the code implicitly recognises only real packages, and
discards virtual packages, because of the heuristic used to recognise
whether a symbol is a package:

  - for real package:
    - symbols  : BR2_PACKAGE_FOO
    - .mk files: foo.mk
  - for virtual packages:
    - symbols  : BR2_PACKAGE_HAS_FOO
    - .mk files: foo.mk

The current heuristic is to check for each symbol if a corresponding .mk
file exists, by stripping 'BR2_PACKAGE_' from the beginning of the symbol,
converting the result to lowercase, and checking if a .mk file exists.

So, as a side effect, it completely misses the virtual packages [*], which
is pretty nice since we get a list with only real packages that the user
can indeed select and see in the menuconfig.

[*] Except for 'cryptodev' and 'jpeg' which are both virtual packages and
normal packages. Except they are not normal packages, they are used to
display a choice of the implementation to use. This case will be fixed in
follow-up patches.

Since we'll soon need to also output the table of virtual packages, we
need to teach the _is_package() function to recognise them as well.

This patch is the first step into that direction: it introduces a new
function _is_real_package() that is just a wrapper to _is_package(), which
gains a new parameter, being the type of packages to filter on.

No behavioural change is made in this patch, it is just a preparatory
patch.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>