pkg-infra: assign BR_NO_CHECK_HASH_FOR so it is recursively-expanded

Currently, assigning to BR_NO_CHECK_HASH_FOR but referencing variable
assigned to later, fails. Here's a failing test-case, which is the
reduced test-case of how we handle BR_NO_CHECK_HASH_FOR for now (warning
added for test-case):

    export BR_NO_CHECK_HASH_FOR
    BR_NO_CHECK_HASH_FOR += $(XENOMAI_SOURCE)
    XENOMAI_VERSION = 2.6.4
    XENOMAI_SOURCE = xenomai-$(XENOMAI_VERSION).tar.bz2
    $(warning BR_NO_CHECK_HASH_FOR='$(BR_NO_CHECK_HASH_FOR)')
    all:

Run it with simply make:

    $ make
    BR_NO_CHECK_HASH_FOR=' '

Now, change the first line to read:

    export BR_NO_CHECK_HASH_FOR =

And we now get:

    $ make
    BR_NO_CHECK_HASH_FOR=' xenomai-2.6.4.tar.bz2'

This new behaviour will be needed later for Xenomai, which handles the
version string in an unusual way, so we can exclude its custom versions.

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