linux-dtb: strip DTB extension properly in postinst/postrm

linux-dtb: strip DTB extension properly in postinst/postrm

The use of awk -F "." in do_install/do_deploy to strip filename extension
was deprecated long time ago in 72980d5bb465f0640ed451d1ebb9c5d2a210ad0c.

Make a similar change in postinst/postrm to properly use basename command.

Otherwise DTB files that contain dots in the name result in broken symlinks
that point to non-existent truncated files.

(From OE-Core rev: ed0f9cf63cb1226f9d92377e13ef63e36a95d29d)

Signed-off-by: Denys Dmytriyenko <denys@ti.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
(From OE-Core rev: 40c2addf0f0ee16b1c1334cf00f1490ffeaac475)

Signed-off-by: Denys Dmytriyenko <denys@ti.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>