Revert "target: remove unused working area 'user' field"

This reverts commit 63a23e6f, which
causes problems with various flash drivers (i.e. LPC1xxx/LPC2xxx)
due to working area handling.

Change-Id: I291ff2ac4f1eef7ecd553d0eb8895b790beb38c3
Signed-off-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-on: http://openocd.zylin.com/872


Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>