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."

I found this problem when testing the k20dx128_bootloader target.  This fixed the problems I was having when trying to update interface code through the bootloader.  Let me know if this should be fixed in some other manner.