component/ca-certificates: don't use implicit `python`

We can not rely on system python here, it's a bit better to rely on
buildout's python ( the main reason is that it complies with
slapos-sr-testing not having system python ).

The implementation is a bit complicated, we can not just reference
${buildout:executable} in the profile, because doing so will cause an
infinite loop with rebootstrap and software installation never finish
because options of ca-certificates change at every run if they include
the python path. Instead, we use a pre-make-hook to rewrite the Makefile
without saving the interpreter in option. This python is only used
during the build process (to generate the certificate files) and not
used at run time, so it does not really depend on python strictly
speaking. Anyway, because it's a component used very early in the
bootstrap, we can not reference python here.

component/ca-certificates/buildout.cfg

@@ -16,10 +16,12 @@ url = http://deb.debian.org/debian/pool/main/c/ca-certificates/ca-certificates_2
 md5sum = e91d3d9259127ba2dbb65fda58d73f31
 patch-binary = ${patch:location}/bin/patch
 patches =
-  ${:_profile_base_location_}/ca-certificates-any-python.patch#b7e234b1b2a40750c054c06603fb3749
+  ${:_profile_base_location_}/ca-certificates-any-python.patch#0f9dfaed4672e53d66f4aebee030d0fd
   ${:_profile_base_location_}/ca-certificates-sbin-dir.patch#0b4e7d82ce768823c01954ee41ef177b
 patch-options = -p0
 configure-command = true
 make-targets = install DESTDIR=@@LOCATION@@ CERTSDIR=certs SBINDIR=sbin
 environment =
   PATH=${xz-utils:location}/bin:%(PATH)s
+pre-make-hook =
+  ${:_profile_base_location_}/ca-certificates-pre-make-hook.py#9e2f6f22d91ea7a089f0ea2c523b0c1e:pre_make_hook

component/ca-certificates/ca-certificates-any-python.patch

@@ -11,13 +11,12 @@
      if line.startswith('CKA_CLASS'):
 --- mozilla/Makefile	2015-12-20 10:49:23.000000000 +0100
 +++ mozilla/Makefile	2016-01-05 20:19:11.006874271 +0100
-@@ -3,7 +3,8 @@
+@@ -3,7 +3,7 @@
  #
  
  all:
 -	python certdata2pem.py
-+	for x in 3 '' 2; do type python$$x && break; done >/dev/null \
-+	&& python$$x certdata2pem.py
++	SLAPOS_BUILDOUT_PYTHON certdata2pem.py
  
  clean:
  	-rm -f *.crt

component/ca-certificates/ca-certificates-pre-make-hook.py

+import pathlib
+import sys
+
+def pre_make_hook(options, buildout, environ):
+  makefile = pathlib.Path('mozilla/Makefile')
+  txt = makefile.read_text().replace('SLAPOS_BUILDOUT_PYTHON', sys.executable)
+  makefile.write_text(txt)