linux: add option to select the CIP real-time kernel

This patch adds Linux CIP RT, the PREEMPT_RT real-time variant maintained
by the CIP team.

Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

linux/Config.in

@@ -53,6 +53,17 @@ config BR2_LINUX_KERNEL_LATEST_CIP_VERSION
 
 	  https://www.cip-project.org
 
+config BR2_LINUX_KERNEL_LATEST_CIP_RT_VERSION
+	bool "Latest CIP RT SLTS version (4.19.72-cip10-rt4)"
+	help
+	  Same as the CIP version, but this is the PREEMPT_RT realtime
+	  variant.
+
+	  The CIP community plans to maintain 4.19 for security and
+	  bug fixes for more than 10 years.
+
+	  https://www.cip-project.org
+
 config BR2_LINUX_KERNEL_CUSTOM_VERSION
 	bool "Custom version"
 	help
@@ -124,6 +135,7 @@ config BR2_LINUX_KERNEL_VERSION
 	string
 	default "5.3.8" if BR2_LINUX_KERNEL_LATEST_VERSION
 	default "4.19.82-cip14" if BR2_LINUX_KERNEL_LATEST_CIP_VERSION
+	default "4.19.72-cip10-rt4" if BR2_LINUX_KERNEL_LATEST_CIP_RT_VERSION
 	default BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE \
 		if BR2_LINUX_KERNEL_CUSTOM_VERSION
 	default "custom" if BR2_LINUX_KERNEL_CUSTOM_TARBALL

linux/linux.mk

@@ -29,7 +29,7 @@ LINUX_SITE_METHOD = hg
 else ifeq ($(BR2_LINUX_KERNEL_CUSTOM_SVN),y)
 LINUX_SITE = $(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_REPO_URL))
 LINUX_SITE_METHOD = svn
-else ifeq ($(BR2_LINUX_KERNEL_LATEST_CIP_VERSION),y)
+else ifeq ($(BR2_LINUX_KERNEL_LATEST_CIP_VERSION)$(BR2_LINUX_KERNEL_LATEST_CIP_RT_VERSION),y)
 LINUX_SOURCE = linux-cip-$(LINUX_VERSION).tar.gz
 LINUX_SITE = https://git.kernel.org/pub/scm/linux/kernel/git/cip/linux-cip.git/snapshot
 else ifneq ($(findstring -rc,$(LINUX_VERSION)),)