setexpr: add regex substring matching and substitution

Add "setexpr name gsub r s [t]" and "setexpr name sub r s [t]"
commands which implement substring matching for the regular
expression <r> in the string <t>, and substitution of the string <s>.
The result is assigned to the environment variable <name>.  If <t> is
not supplied, the previous value of <name> is used instead.  "gsub"
performs global substitution, while "sub" will replace only the first
substring.

Both commands are closely modeled after the gawk functions with the
same names.

Examples:

- Generate broadcast address by substituting the last two numbers of
  the IP address by "255.255":

  	=> print ipaddr
	ipaddr=192.168.1.104
	=> setexpr broadcast sub "(.*\\.).*\\..*" "\\1255.255" $ipaddr
	broadcast=192.168.255.255

- Depending on keyboard configuration (German vs. US keyboard) a
  barcode scanner may initialize the MAC address as C0:E5:4E:02:06:DC
  or as C0>E5>4E>02>06>DC.  Make sure we always have a correct value:

	=> print ethaddr
	ethaddr=C0>E5>4E>02>06>DC
	=> setexpr ethaddr gsub > :
	ethaddr=C0:E5:4E:02:06:DC

- Do the same, but substitute one step at a time in a loop until no
  futher matches:

	=> setenv ethaddr C0>E5>4E>02>06>DC
	=> while setexpr ethaddr sub > :
	> do
	> echo -----
	> done
	ethaddr=C0:E5>4E>02>06>DC
	-----
	ethaddr=C0:E5:4E>02>06>DC
	-----
	ethaddr=C0:E5:4E:02>06>DC
	-----
	ethaddr=C0:E5:4E:02:06>DC
	-----
	ethaddr=C0:E5:4E:02:06:DC
	-----
	C0:E5:4E:02:06:DC: No match
	=> print ethaddr
	ethaddr=C0:E5:4E:02:06:DC

etc.

To enable this feature, the CONFIG_REGEX option has to be defined in
the board config file.

Signed-off-by: Wolfgang Denk <wd@denx.de>