Merge manifest into the board DT, add boot flow drivers

Hafnium currently supports two kinds of boot flow:
(a) Linux-like, where the board FDT is passed by the boot loader to
the OS kernel in first kernel arg (this is used by QEMU, FVP, RPi, ...),
(b) Android-like, where the FDT used by Hafnium is compiled into it and
the VMs have their own.

This used to be implemented using weak symbols and each board spec
overriding it with its own implementation. This patch introduces the
concept of boot-flow drivers in the main tree, and the corresponding
driver selected using a config option in board spec. Drivers for the two
boot-flows described above are implemented.

Simultaneously, the manifest is now read from the FDT available at
Hafnium init time. This required two notable changes:
(1) all values are copied into the manifest struct because FDT is
unmapped, modified and passed to the primary VM,
(2) manifest is now written as an overlay; QEMU and FVP test drivers
overlay it automatically.

Bug: 117551352
Change-Id: Ieae7fe4ef5b3047174ec0ad057e487660ccd5a03
diff --git a/docs/Manifest.md b/docs/Manifest.md
new file mode 100644
index 0000000..58a5600
--- /dev/null
+++ b/docs/Manifest.md
@@ -0,0 +1,79 @@
+# Hafnium Manifest
+
+## Format
+
+The format of the manifest is a simple DeviceTree overlay:
+
+```
+/dts-v1/;
+/plugin/;
+
+&{/} {
+	hypervisor {
+		vm1 {
+			debug_name = "name";
+		};
+
+		vm2 {
+			debug_name = "name";
+			kernel_filename = "filename";
+			vcpu_count = <N>;
+			mem_size = <M>;
+		};
+		...
+	};
+};
+```
+
+Note: `&{/}` is a syntactic sugar expanded by the DTC compiler. Make sure to
+use the DTC in `prebuilts/` as the version packaged with your OS may not support
+it yet.
+
+## Example
+
+The following manifest defines two secondary VMs, the first one with 1MB of
+memory, 2 CPUs and kernel image called `kernel0` (matches filename in Hafnium's
+[ramdisk](HafniumRamDisk.md)), while the second one has 2MB of memory, 4 CPUs
+and a kernel image called `kernel1`.
+
+```
+/dts-v1/;
+/plugin/;
+
+&{/} {
+	hypervisor {
+		vm1 {
+			debug_name = "primary VM";
+		};
+
+		vm2 {
+			debug_name = "secondary VM 1";
+			kernel_filename = "kernel0";
+			vcpu_count = <2>;
+			mem_size = <0x100000>;
+		};
+
+		vm3 {
+			debug_name = "secondary VM 2";
+			kernel_filename = "kernel1";
+			vcpu_count = <4>;
+			mem_size = <0x200000>;
+		};
+	};
+};
+```
+
+## Compiling
+
+Hafnium expects the manifest as part of the board FDT, i.e. DeviceTree in binary
+format (DTB).
+
+First, compile the manifest into a DTBO (binary overlay):
+```shell
+prebuilts/linux-x64/dtc/dtc -I dts -O dtb --out-version 17 -o manifest.dtbo <manifest_source_file>
+```
+
+Then overlay it with the DTB of your board:
+```shell
+prebuilts/linux-x64/dtc/fdtoverlay -i <board DTB> -o <output DTB> manifest.dtbo
+```