conf/machine: Make preferred virtual/kernel overridable

A user of the BSP layer may decide to use the machine
configuration but a different kernel.

Signed-off-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm-bsp/conf/machine/a5ds.conf

@@ -6,7 +6,7 @@
 
 require conf/machine/include/tune-cortexa5.inc
 
-PREFERRED_PROVIDER_virtual/kernel = "linux-upstream-arm"
+PREFERRED_PROVIDER_virtual/kernel ?= "linux-upstream-arm"
 PREFERRED_VERSION_linux-upstream-arm ?= "5.3%"
 KBUILD_DEFCONFIG = "multi_v7_defconfig"
 

meta-arm-bsp/conf/machine/fvp-common/fvp.inc

@@ -34,7 +34,7 @@ DISK_IMG_PARTITION3_CONTENT = ""
 
 SERIAL_CONSOLES = "115200;ttyAMA0"
 
-PREFERRED_PROVIDER_virtual/kernel = "linux-yocto"
+PREFERRED_PROVIDER_virtual/kernel ?= "linux-yocto"
 PREFERRED_VERSION_linux-yocto ?= "4.19%"
 
 EXTRA_IMAGEDEPENDS += "virtual/trusted-firmware-a u-boot"

meta-arm-bsp/conf/machine/gem5-arm64.conf

@@ -19,7 +19,7 @@ SERIAL_CONSOLES = "115200;ttyAMA0"
 
 EXTRA_IMAGEDEPENDS += "virtual/gem5-bootloader"
 
-PREFERRED_PROVIDER_virtual/kernel = "linux-yocto"
+PREFERRED_PROVIDER_virtual/kernel ?= "linux-yocto"
 PREFERRED_VERSION_linux-yocto ?= "4.19%"
 
 

meta-arm-bsp/conf/machine/juno.conf

@@ -18,7 +18,7 @@ IMAGE_FSTYPES += "tar.bz2 ext4"
 SERIAL_CONSOLES = "115200;ttyAMA0"
 
 # Use kernel provided by linaro (Contains support for SCMi or HDMI)
-PREFERRED_PROVIDER_virtual/kernel = "linux-linaro-arm"
+PREFERRED_PROVIDER_virtual/kernel ?= "linux-linaro-arm"
 PREFERRED_VERSION_linux-linaro-arm ?= "4.19%"
 
 EXTRA_IMAGEDEPENDS += "virtual/trusted-firmware-a u-boot"