[build-script] Create 'buildbot_linux' preset to reduce duplication

The Ubuntu 14.04 preset existed to skip building Foundation due to
unavailable ICU versions. The problem was corrected, but the
comment and preset duplication of 'buildbot_linux_*' in
build-presets.ini remained.

This change adds a 'buildbot_linux' preset as the primary Linux
preset and changes 'buildbot_linux_*' to use it as a mixin.

Implemented as discussed with Dmitri Gribenko on swift-dev.

Author: 13rac1

utils/build-presets.ini

@@ -506,8 +506,7 @@ install-destdir=%(install_destdir)s
 # Path to the .tar.gz package we would create.
 installable-package=%(installable_package)s
 
-# This preset builds foundation
-[preset: buildbot_linux_1510]
+[preset: buildbot_linux]
 mixin-preset=mixin_linux_installation
 build-subdir=buildbot_linux
 lldb
@@ -521,20 +520,13 @@ dash-dash
 install-foundation
 reconfigure
 
-# This preset does not build Foundation due to unavailable ICU versions on Ubuntu 14.04
-[preset: buildbot_linux_1404]
-mixin-preset=mixin_linux_installation
-build-subdir=buildbot_linux
-lldb
-release
-test
-validation-test
-foundation
-
-dash-dash
+# Ubuntu 15.10 preset for backwards compat and future customizations.
+[preset: buildbot_linux_1510]
+mixin-preset=buildbot_linux
 
-install-foundation
-reconfigure
+# Ubuntu 14.04 preset for backwards compat and future customizations.
+[preset: buildbot_linux_1404]
+mixin-preset=buildbot_linux
 
 #===------------------------------------------------------------------------===#
 # OS X Package Builders
@@ -619,7 +611,7 @@ installable-package=%(installable_package)s
 # Path to the .tar.gz symbols package
 symbols-package=%(symbols_package)s
 
-# Info.plist 
+# Info.plist
 darwin-toolchain-bundle-identifier=%(darwin_toolchain_bundle_identifier)s
 darwin-toolchain-display-name=%(darwin_toolchain_display_name)s
 darwin-toolchain-name=%(darwin_toolchain_xctoolchain_name)s