cortex_m: set fpb_enabled on enabling fpb

cortex_m: set fpb_enabled on enabling fpb

If the fpb_enabled is not set then as part of cortex_m3_set_breakpoint we
enable the fpb, however we do not signal the fpb as being enabled.

This issue only effects the hla target as the current cortex_m code enables
the fpb during cortex_m3_endreset_event.

Change-Id: I44d3fc65916c131b7a226869dd16aed5afb441b4
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1634
Tested-by: jenkins