merge_dtbs.py: Print only basenames of dtb* files

When debugging issues with the script, it helps to not have in the
script's output the fully qualified path names of the DTB and DTBO files
being merged as they are very lengthy. Retain only the relevant parts of
the filnames - the basenames for better readability.

Change-Id: Ia18584fcf9ff173c4b22d4543c83c4091e15b8f8
Signed-off-by: Guru Das Srinagesh <quic_gurus@quicinc.com>