buildroot: make mounting /boot optional

Currently, we take to paths from the root file system supplied
to the `BuildRoot` class: `/boot` and `/usr`. The reason for
mounting `/boot` is that grub2 and shim install efi binaries
there and for certain images we want to copy the binaries from
the build root and not install the respective packages.
However, if we build to build root itself, we probably don't
want the mount the hosts' `/boot` since we don't want to copy
anything from there. This change should give us the ability to
do exactly that.

osbuild/buildroot.py

@@ -69,6 +69,7 @@ class BuildRoot(contextlib.AbstractContextManager):
         self._apis = []
         self.dev = None
         self.var = None
+        self.mount_boot = True
 
     @staticmethod
     def _mknod(path, name, mode, major, minor):
@@ -154,7 +155,11 @@ class BuildRoot(contextlib.AbstractContextManager):
         mounts = []
 
         # Import directories from the caller-provided root.
-        for p in ["boot", "usr"]:
+        imports = ["usr"]
+        if self.mount_boot:
+            imports.insert(0, "boot")
+
+        for p in imports:
             source = os.path.join(self._rootdir, p)
             if os.path.isdir(source) and not os.path.islink(source):
                 mounts += ["--ro-bind", source, os.path.join("/", p)]