Provide option to patch static kernel config file

This is useful in certain scenarios where we want to build a kernel
based off an existing config file but also apply slightly different
values during compilation. It doesnt always make sense to keep
maintaining lists of static configs for each set of tweaks. This commit
allows one to pass in an ansible variable file filled with config
directives.

defaults/main.yml

@@ -95,3 +95,13 @@ grsecurity_build_gpg_keys_ubuntu:
     fingerprint: DB5D7CCAF3994E3395DA4D3EE8675DEECBEECEA3
   - name: Thadeu Lima de Souza Cascardo (Canonical)
     fingerprint: 279357DB6127376E6D1DF1BCAAD56799FBFD0D3E
+
+# Apply additional file of kernel configuration options
+# to be applied to an existing configuration. Format of the file will be
+# yaml with each directive per-line. An example might look like:
+#
+# CONFIG_HYPERVISOR_GUEST: y
+# CONFIG_PARAVIRT: y
+# CONFIG_XEN: y
+# CONFIG_XEN_DOM0: y
+grsecurity_build_custom_config_applydiff: ''

tasks/configure.yml

@@ -7,6 +7,20 @@
     src: "{{ grsecurity_build_custom_config if grsecurity_build_custom_config != '' else 'config-'+grsecurity_build_strategy }}"
     dest: "{{ grsecurity_build_linux_source_directory }}/.config"
 
+- name: Load up kernel arguments from provided vars file
+  include_vars:
+    file: "{{ grsecurity_build_custom_config_applydiff }}"
+    name: _grsec_patch_config
+  when: grsecurity_build_custom_config_applydiff != ''
+
+- name: Optionally patch the baseline config
+  lineinfile:
+    line: "{{ item.key }}={{ item.value }}"
+    regexp: "^{{ item.key }}="
+    dest: "{{ grsecurity_build_linux_source_directory }}/.config"
+  with_dict: "{{ _grsec_patch_config|default({}) }}"
+  when: grsecurity_build_custom_config_applydiff != ''
+
 - name: Ensure any new options are updated with defaults.
   command: make olddefconfig
   args: