build: envkernel: overmount outdir/Makefile to use local include (MR 2504)

the kernel has pending patches in -next which adjust the generated
Makefile in the output directory to include the source tree makefile
with an absolute path, this breaks envkernel which relies on it being a
relative path.

Fix this by mounting our own Makefile instead using the relative path.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>

pmb/build/envkernel.py

@@ -119,6 +119,20 @@ def modify_apkbuild(pkgname: str, aport: Path) -> None:
     pmb.aportgen.core.rewrite(pkgname, apkbuild_path, fields=fields)
 
 
+def overmount_makefile(context: Context, kbuild_out: str) -> None:
+    makefile = context.config.work / "tmp_kernel.mk"
+    makefile.open("w").write("""
+# Automatically generated by pmbootstrap to use relative include paths
+include ../Makefile
+""")
+
+    makefile_target = Chroot.native() / "mnt/linux" / kbuild_out / "Makefile"
+
+    logging.info("Overmounting Makefile to use relative include paths")
+    if not pmb.helpers.mount.ismount(makefile_target):
+        pmb.helpers.mount.bind_file(makefile, makefile_target)
+
+
 def run_abuild(
     context: Context, pkgname: str, arch: Arch, apkbuild_path: Path, kbuild_out: str
 ) -> None:
@@ -141,6 +155,9 @@ def run_abuild(
 
     pmb.helpers.mount.bind(Path("."), chroot / "mnt/linux")
 
+    if kbuild_out:
+        overmount_makefile(context, kbuild_out)
+
     if not os.path.exists(chroot / kbuild_out_source):
         raise RuntimeError(
             "No '.output' dir found in your kernel source dir. "
@@ -180,7 +197,7 @@ def run_abuild(
     )
 
     # Create symlink from abuild working directory to envkernel build directory
-    pmb.chroot.root(["ln", "-sf", "/mnt/linux", build_path / "src"])
+    pmb.chroot.root(["ln", "-sf", kbuild_out_source, build_path / "src"])
 
     cmd: list[PathString] = ["cp", apkbuild_path, chroot / build_path / "APKBUILD"]
     pmb.helpers.run.root(cmd)