common: Mark output from Infix shell scripts similarly to Make

Similarly to how $(call IXMSG,"hello world") outputs a blue
information line from a makefile, provide `ixmsg` to accomplish the
same thing in shell scripts.

Use a different prefix (`#!:` vs `>>>`) to differentiate it from
makefile output.

board/common/lib.sh

@@ -1,3 +1,8 @@
+ixmsg()
+{
+    printf "\e[37;44m#!:   $@\e[0m\n"
+}
+
 die()
 {
     echo "$@" >&2

board/common/post-image.sh

@@ -7,19 +7,24 @@ load_cfg BR2_EXTERNAL_INFIX_PATH
 
 load_cfg BR2_ARCH
 load_cfg SIGN_KEY
+
+ixmsg "Signing SquashFS Image"
 $common/sign.sh $BR2_ARCH $SIGN_KEY
 
 load_cfg DISK_IMAGE
 if [ "$DISK_IMAGE" = "y" ]; then
+    ixmsg "Creating Disk Image"
     $common/mkdisk.sh -a $BR2_ARCH
 fi
 
 load_cfg GNS3_APPLIANCE
 if [ "$GNS3_APPLIANCE" = "y" ]; then
+    ixmsg "Creating GNS3 Appliance"
     $common/mkgns3a.sh
 fi
 
 load_cfg FIT_IMAGE
 if [ "$FIT_IMAGE" = "y" ]; then
+    ixmsg "Creating Traditional FIT Image"
     $common/mkfit.sh
 fi