gdb_server: correctly report flash sector sizes

gdb_server: correctly report flash sector sizes

Report each region of same-size sectors separately, instead of
incorrectly reporting that every sector has the same size.

This is a longstanding bug on NOR flash chips with non-uniform
sector sizes.  It was largely hidden by other bugs in flash
handling.  When some of those were recently fixed, this one was
exposed as a regression on str710.

[oyvind.harboe@zylin.com: update the loop to behave on str7 ]

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>