Fix serious bug in LPC2xxx/LPC17xx flash algorithm.

Fix serious bug in LPC2xxx/LPC17xx flash algorithm.

Flash algorithm for LPC17xx/LPC2xxx was trying to "reuse" previously
allocated working area on next flashing which is not possible -
working areas are freed automatically on reset. This caused all but
first flashing attempts to fail. As there is no point in storing pointer
to working area, it was converted to local variable.

Change-Id: I939946325ff9eecc4861c0f51ab0f73871a3d7b9
Signed-off-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-on: http://openocd.zylin.com/860
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>