shell: update HISTFILESIZE code to be actually useful

"HISTFILESIZE=0" in profile wasn't working as intended,
"unset HISTFILE" wasn't preventing creation of history files
Now:
HISTSIZE=n      allows to reduce in-memory history buffer
HISTFILESIZE=n  allows to reduce history file size (0: truncate it)
unset HISTFILE  allows to not save history file at all

function                                             old     new   delta
exitshell                                            138     194     +56
hush_exit                                             97     143     +46
save_history                                         266     296     +30
hush_main                                           1170    1186     +16
.rodata                                           105762  105771      +9
load_history                                         246     254      +8
size_from_HISTFILESIZE                                44      41      -3
read_line_input                                     2746    2712     -34
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 6/2 up/down: 165/-37)           Total: 128 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/shell/Config.src b/shell/Config.src
index 5efbf9995..5b3fe08f3 100644
--- a/shell/Config.src
+++ b/shell/Config.src
@@ -166,9 +166,10 @@ config FEATURE_SH_HISTFILESIZE
         default y
         depends on SHELL_ASH || SHELL_HUSH
         help
-        This option makes busybox shells to use $HISTFILESIZE variable
-        to set shell history size. Note that its max value is capped
-        by "History size" setting in library tuning section.
+        This option makes busybox shells to use $HISTSIZE and
+        $HISTFILESIZE variables to set shell history size.
+        Note that its max value is capped by "History size" setting
+        in library tuning section.
 
 config FEATURE_SH_EMBEDDED_SCRIPTS
         bool "Embed scripts in the binary"