Disable kernel compatibility check for mainline kernel.

Test: vts_treble_vintf_framework_test --gtest_filter=*KernelCompatibility
Bug: 153359371
Change-Id: I88fcbe6ea5aac679a0f6239d2dbfa88e21b3e40c
diff --git a/RuntimeInfo-target.cpp b/RuntimeInfo-target.cpp
index d221897..8889b03 100644
--- a/RuntimeInfo-target.cpp
+++ b/RuntimeInfo-target.cpp
@@ -17,6 +17,7 @@
 
 #define LOG_TAG "libvintf"
 #include <android-base/logging.h>
+#include <android-base/strings.h>
 
 #include "RuntimeInfo.h"
 
@@ -40,6 +41,8 @@
 #define PROC_CONFIG "/proc/config.gz"
 #define BUFFER_SIZE sysconf(_SC_PAGESIZE)
 
+static constexpr char kMainline[] = "-mainline-";
+
 namespace android {
 namespace vintf {
 
@@ -144,6 +147,8 @@
     if (!parse(mRuntimeInfo->mOsRelease.substr(0, pos), &mRuntimeInfo->mKernel.mVersion)) {
         return UNKNOWN_ERROR;
     }
+    mRuntimeInfo->mIsMainline =
+        android::base::StartsWith(mRuntimeInfo->mOsRelease.substr(pos), kMainline);
     return OK;
 }