boot/uboot: cover more u-boot versions when fixing up libfdt include paths

boot/uboot: cover more u-boot versions when fixing up libfdt include paths

The original UBOOT_FIXUP_LIBFDT_INCLUDE hook only works for u-boot versions
starting from v2017.11.

As older u-boot versions are still in common use today, e.g. in the
ci20_defconfig, extend the hook to cover a wider range of u-boot versions.

The main code change was proposed by Arnout Vandecappelle in [1].
Testing and comment changes were done by Thomas De Schampheleire.
Additional (build) testing done by Yann.

[1] http://lists.busybox.net/pipermail/buildroot/2019-May/251231.html

This was tested with the following defconfigs, that each excercise one
case:

    ----.--     No defconfig identified
    2010.06     'lib/libfdt/', used to be simply 'libfdt' before upstream commit 0de71d507157...
    2013.10     ci20_defconfig
    2014.04     'srctree' used to be called 'SRCTREE' before upstream commit 01286329b27b27ea...
    2017.05     olimex_a13_olinuxino_defconfig
    2017.11     scripts/dtc/libfdt only exists since upstream commit c0e032e0090d6541549b19cc...
    2018.01     mx6udoo_defconfig
    2018.03     core change is equivalent to upstream commit e0d20dc1521e74b82dbd69be53a04884...
    2019.10     nanopi_neo_defconfig

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
[yann.morin.1998@free.fr: add the build-test results]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>