Fix Kinetis EraseSector function

The size to erase passed into flash_erase by EraseSector is 1 which
is invalid.  The function flash_check_range will return an error which
in turn will cause flash sector erase to fail.  This patch replaces
the fixed sector erase size of 1 with g_flash.PFlashSectorSize which
is a valid size.  This fixes interface updates from the bootloader.
This functionality was initially broken by the commit
f3e5f3cce177cfe69fd1b0417227026c3d114004 -
"Cleaned up target_flash.inc files."
Merge pull request #61 from c1728p9/master

Fix Kinetis EraseSector function