aportgen linux: always override KBUILD_OUTPUT (#1556)

Some kernels have a different `KBUILD_OUTPUT` path (e.g. #1551). When
the output path is different from `$srcdir/build`, then
`pmbootstrap kconfig edit` will not work (same with the previous
`pmbootstrap menuconfig` implementation).

This commit forces the output path to be `$srcdir/build` in the template
for new kernel aports, so we won't have that issue with future ports.

pmb/aportgen/linux.py

@@ -110,8 +110,9 @@ def generate_apkbuild(args, pkgname, deviceinfo):
             done
 
             # Prepare kernel config ('yes ""' for kernels lacking olddefconfig)
+            mkdir -p "$srcdir"/build
             cp "$srcdir"/$_config "$builddir"/.config
-            yes "" | make ARCH="$_carch" HOSTCC="$HOSTCC" oldconfig
+            yes "" | make O="$srcdir"/build ARCH="$_carch" HOSTCC="$HOSTCC" oldconfig
         }
 
         build() {""" + build + """