Implement kexec

To support ramdump of VMs, implement a kexec binary and add it to
Microdroid. The binary will be used to preload the kernel and the init
ramdisk that the system will boot into when a kernel panic occurs.

The binary is very minimal. It has much less functionalities than the
upstream kexec-tools. It has a different cmdline interface, and is just
a thin wrapper around the kexec_file_load syscall. It supports loading
the kernel and the ramdisk only for the exec-on-crash use case.

Bug: 237380224
Test: run kexec on Microdroid
Change-Id: I75610d4f55c97a79a7200ecf9e8aa7fc7b12c010
diff --git a/microdroid/kdump/Android.bp b/microdroid/kdump/Android.bp
new file mode 100644
index 0000000..1a8c9cc
--- /dev/null
+++ b/microdroid/kdump/Android.bp
@@ -0,0 +1,10 @@
+package {
+    default_applicable_licenses: ["Android-Apache-2.0"],
+}
+
+cc_binary {
+    name: "microdroid_kexec",
+    stem: "kexec",
+    srcs: ["kexec.c"],
+    installable: false,
+}