Add command to erase all flash memory regions

Add command to erase all flash memory regions

Changes in v4:

- Replaced phex call with hex_string.

Changes in v3:

- Addressed comments by Pedro.
- Output of memory region size now in hex format.
- Misc formatting fixups.
- Addressed Simon's comments on formatting.
- Adjusted command text in the manual entry.
- Fixed up ChangeLog.
- Renamed flash_erase_all_command to flash_erase_command.

Changes in v2:

- Added NEWS entry.
- Fixed long lines.
- Address printing with paddress.

Years ago we contributed flash programming patches upstream.  The following
patch is a leftover one that complements that functionality by adding a new
command to erase all reported flash memory blocks.

The command is most useful when we're dealing with flash-enabled targets
(mostly bare-metal) and we need to reset the board for some reason.

The wiping out of flash memory regions should help the target come up with a
known clean state from which the user can load a new image and resume
debugging. It is convenient enough to do this from the debugger, and there is
also an MI command to expose this functionality to the IDE's.

gdb/doc/ChangeLog:

2017-01-20  Mike Wrighton  <mike_wrighton@codesourcery.com>
	    Luis Machado  <lgustavo@codesourcery.com>

	* gdb.texinfo (-target-flash-erase): New MI command description.
	(flash-erase): New CLI command description.

gdb/ChangeLog:

2017-01-20  Mike Wrighton  <mike_wrighton@codesourcery.com>
	    Luis Machado  <lgustavo@codesourcery.com>

	* NEWS (New commands): Mention flash-erase.
	(New MI commands): Mention target-flash-erase.
	* mi/mi-cmds.c (mi_cmd_target_flash_erase): Add target-flash-erase MI
	command.
	* mi/mi-cmds.h (mi_cmd_target_flash_erase): New declaration.
	* mi/mi-main.c (mi_cmd_target_flash_erase): New function.
	* target.c (flash_erase_command): New function.
	(initialize_targets): Add new flash-erase command.
	* target.h (flash_erase_command): New declaration.