e2fsprogs/populate-extfs.sh: fix a problem on dash

e2fsprogs/populate-extfs.sh: fix a problem on dash

The dash can't handle the or [[ in parameter expansion, for example:

A=/usr/bin/[[
B=[[
C="${A%$B}"

The C should be "/usr/bin" in common, but it will be /usr/bin/[[ on
dash, use dirname to fix it.

NOTE:
There are 3 lines about parameter expansion, only fix the
DIR="${DIR%$TGT}" since the other 2 works will and are very useful in
this case.

[YOCTO #5712]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>