risc-v: Pin oldest kernel to 4.15 for the new kid

(From OE-Core rev: fd3d98fd5690dab391c27a71e26f74e0f9a25f3a)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/conf/bitbake.conf

@@ -423,6 +423,8 @@ SDKPATHNATIVE = "${SDKPATH}/sysroots/${SDK_SYS}"
 OLDEST_KERNEL = "3.2.0"
 OLDEST_KERNEL_aarch64 = "3.14"
 OLDEST_KERNEL_nios2 = "3.19"
+OLDEST_KERNEL_riscv32 = "4.15"
+OLDEST_KERNEL_riscv64 = "4.15"
 
 # SDK_OLDEST_KERNEL can't be set using overrides since there are
 # none for the SDK architecture. Best to set it from a machine-sdk