dfu: thor: fix: Modify dfu_get_alt() function to support absolute paths

Recently the ext4 file system imposed passing absolute path with its file
name parameter.
As a result dfu_alt_info env variable has been modified to provide absolute
path when ext4 file system is accessed (e.g. /uImage ext4 0 2;).

Unfortunately, lthor flashing program provides plain file name (like uImage)
and hence those two file names do not match anymore.

Presented commit also allows lthor to write files to sub directories (like
/boot/bin/uImage).

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Reviewed-by: Marek Vasut <marex@denx.de>