devtool: deploy-target: Avoid unnecessary dependency on awk on the target

devtool: deploy-target: Avoid unnecessary dependency on awk on the target

Relying on that awk is installed on the target just to extract the
fourth column (i.e., the free volume size) from `df -P` is an
unnecessary dependency for devtool deploy-target. As it is already
using sed to mangle the output from `df -P`, this can easily be
modified to only extract the free volume size.

(From OE-Core rev: 7bab454b0bf0075fbb2a5de06286a9da1df2adc6)

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>