Add setexpr command

Add a simple expr style command that will set an env variable as the result
of the command.  This allows us to do simple math in shell.  The following
operations are supported: &, |, ^, +, -, *, /.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
diff --git a/common/Makefile b/common/Makefile
index 56d0581..56176ca 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -88,6 +88,7 @@
 COBJS-$(CONFIG_CMD_REISER) += cmd_reiser.o
 COBJS-y += cmd_sata.o
 COBJS-$(CONFIG_CMD_SCSI) += cmd_scsi.o
+COBJS-$(CONFIG_CMD_SETEXPR) += cmd_setexpr.o
 COBJS-$(CONFIG_CMD_SPI) += cmd_spi.o
 COBJS-$(CONFIG_CMD_STRINGS) += cmd_strings.o
 COBJS-$(CONFIG_CMD_TERMINAL) += cmd_terminal.o