Move flash persistent state to flash_common

Persistent state is needed by all platforms to hold the
protect-ro-at-boot flag.  STM32F100 and LM4 implementations were
near-identical, and are now common code (with one #ifdef to handle the
single place where they weren't).

STM32L doesn't use pstate yet, but it'll need to.  I can't simply
store the protect-ro-at-boot flag inside the WRP registers themselves
because they're still writable in EC-RW.  The change to STM32L to use
pstate is coming next.

BUG=chrome-os-partner:15613
BRANCH=none
TEST=build pit, link, spring; on link and spring, do

 - flashinfo -> (no flags)
 - enable WP (via screw or dut-control)
 - flashinfo -> wp_gpio_asserted
 - flashwp enable
 - flashinfo -> wp_gpio_asserted ro_at_boot
 - flashwp now
 - flashinfo -> wp_gpio_asserted ro_at_boot all_now (and possibly ro_now)
 - flashwp disable -> fails
 - flashinfo -> wp_gpio_asserted ro_at_boot all_now
 - reboot ap-off
 - flashinfo -> wp_gpio_asserted ro_at_boot ro_now
 - disable WP (via screw or dut-control)
 - reboot
 - flashinfo -> ro_at_boot
 - flashwp disable
 - flashinfo -> (no flags)

(Note that on Spring you'll need to 'forceen on' before enabling WP,
or the console will be disabled once you enable ro_at_boot and reboot.)

Change-Id: I415388b98ec8bf1d149803aaaa7fe8c7f3076c36
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/56064

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