boot-directdisk: use awk instead of echo -ne for dash compatibility

boot-directdisk: use awk instead of echo -ne for dash compatibility

The echo command is a dash shell built-in and does not support the
-n and -e options so use awk instead.

This fixes the partition table being corrupted when writing the MBR
disk signature into the image.

[YOCTO #4859]

Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>