- add flash_eraseall

- add flash_eraseall
This is the result after converting mtd-utils' flash_eraseall to BB.
The functionality given by this patch almost the same except that this
one does not support long options.
I needed this tool a system which does not have a lot of flash for RFS
and merging this into BB as the only way out.

[bigeasy@]$ ./scripts/bloat-o-meter bb_wo_fl bb_w_fl
function                                             old     new   delta
flash_eraseall_main                                    -    1072   +1072
show_progress                                          -      62     +62
packed_usage                                       25156   25176     +20
applet_names                                        1958    1973     +15
applet_main                                         2352    2360      +8
target_endian                                          -       4      +4
applet_nameofs                                       588     590      +2
applet_install_loc                                   147     148      +1
------------------------------------------------------------------------------
(add/remove: 3/0 grow/shrink: 5/0 up/down: 1184/0)           Total: 1184 bytes
[bigeasy@]$ size miscutils/flash_eraseall.o
   text    data     bss     dec     hex filename
   1586       4       0    1590     636 miscutils/flash_eraseall.o

Signed-off-by: Benedigt Spranger <b.spranger@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>