[source] build: allow specifying flow-control to grub on serial console

[LEDE Commits]

nbd pushed a commit to source.git, branch master:
https://git.lede-project.org/7fe5963be023fd59a11652ecfb065d3405a91c48

commit 7fe5963be023fd59a11652ecfb065d3405a91c48
Author: Philip Prindeville <philipp at redfish-solutions.com>
AuthorDate: Mon Feb 6 20:33:36 2017 -0700

    build: allow specifying flow-control to grub on serial console
    
    On the more sophisticated (i.e. deeper FIFO) serial controllers,
    flow-control might be needed to avoid dropping output.
    
    Signed-off-by: Philip Prindeville <philipp at redfish-solutions.com>
---
 config/Config-images.in         | 5 +++++
 target/linux/x86/image/Makefile | 4 ++--
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/config/Config-images.in b/config/Config-images.in
index f59a336..95752f7 100644
--- a/config/Config-images.in
+++ b/config/Config-images.in
@@ -203,6 +203,11 @@ menu "Target Images"
 		default 38400 if TARGET_x86_generic
 		default 115200
 
+	config GRUB_FLOWCONTROL
+		bool "Use RTE/CTS on serial console"
+		depends on GRUB_SERIAL != ""
+		default n
+
 	config GRUB_BOOTOPTS
 		string "Extra kernel boot options"
 		depends on GRUB_IMAGES
diff --git a/target/linux/x86/image/Makefile b/target/linux/x86/image/Makefile
index f34788d..ca5d012 100644
--- a/target/linux/x86/image/Makefile
+++ b/target/linux/x86/image/Makefile
@@ -32,8 +32,8 @@ endif
 GRUB_SERIAL:=$(call qstrip,$(CONFIG_GRUB_SERIAL))
 
 ifneq ($(GRUB_SERIAL),)
-  GRUB_CONSOLE_CMDLINE += console=$(GRUB_SERIAL),$(CONFIG_GRUB_BAUDRATE)n8
-  GRUB_SERIAL_CONFIG := serial --unit=0 --speed=$(CONFIG_GRUB_BAUDRATE) --word=8 --parity=no --stop=1 --rtscts=off
+  GRUB_CONSOLE_CMDLINE += console=$(GRUB_SERIAL),$(CONFIG_GRUB_BAUDRATE)n8$(if $(CONFIG_GRUB_FLOWCONTROL),r,)
+  GRUB_SERIAL_CONFIG := serial --unit=0 --speed=$(CONFIG_GRUB_BAUDRATE) --word=8 --parity=no --stop=1 --rtscts=$(if $(CONFIG_GRUB_FLOWCONTROL),on,off)
   GRUB_TERMINALS += serial
 endif