flash: Only erase flash block that contain data

It wastes time to erase blocks that are already erased and it is faster
on stm32 to check first. Add a check in flash_physical_erase() on all
chips, using a common flash_is_erased() function.

BUG=none
BRANCH=snow,link
TEST=manual
Do software sync in U-Boot and see that it succeeds. This tests that
we can still erase and then boot a written image. It typically saves
a second on a full sync over i2c.

SMDK5250 # cros_test swsync -f
SF: Detected W25Q32 with page size 4 KiB, total 4 MiB
Flashing RW EC image: erasing, writing, done
Flashing RO EC image: erasing, writing, done
Full software sync completed in 22.949s
SMDK5250 #

Also see that second erase is faster:

SMDK5250 # time mkbp erase rw

time: 0.952 seconds, 952 ticks
SMDK5250 # time mkbp erase rw

time: 0.054 seconds, 54 ticks
SMDK5250 #

Change-Id: I3699577217fdbb2f212d20d150d3ca15fdff03eb
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/30851
Reviewed-by: Randall Spangler <rspangler@chromium.org>

include/flash.h

@@ -31,6 +31,17 @@ static inline char *flash_physical_dataptr(int offset)
 	return (char *)(CONFIG_FLASH_BASE + offset);
 }
 
+/**
+ * Check if a region of flash is erased
+ *
+ * It is assumed that an erased region has all bits set to 1.
+ *
+ * @param offset	Flash offset to check
+ * @param size		Number of bytes to check (word-aligned)
+ * @return 1 if erased, 0 if not erased
+ */
+int flash_is_erased(uint32_t offset, int size);
+
 /**
  * Write to physical flash.
  *