allow 'flash_banks' command to give GDB output

allow 'flash_banks' command to give GDB output

The 'flash banks' command produces a list that needs to be formatted
properly for GDB's 'mem info' to work properly.  The flash_banks TCL
wrapper provided this formatting, but wrappers no longer work for
second-level commands as they did in the past.  With this patch,
the 'flash_banks' command can be used with the new command syntax
and display the required information.