bitbake: Add task specific stamp file support

This patch, based on proof of concept code from Richard adds code to
bitbake to allow individual tasks to optionally specify their stamp
file using the stamp-base flag. This takes the same form as the STAMP
variable but can be specified on a per task basis.

Code is also added to runqueue to ensure that if two tasks share the
same stamp file, only one will be executed at once.

A significant usecase for this code is to share source code (${S})
between recipes where separate build directories (${B}) are used.

(Bitbake rev: 41bef02bef8379590ba012319aebe05068a8081e)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/cache.py

@@ -124,6 +124,7 @@ class CoreRecipeInfo(RecipeInfoCommon):
         self.broken = self.getvar('BROKEN', metadata)
         self.not_world = self.getvar('EXCLUDE_FROM_WORLD', metadata)
         self.stamp = self.getvar('STAMP', metadata)
+        self.stamp_base = self.flaglist('stamp-base', self.tasks, metadata)
         self.stamp_extrainfo = self.flaglist('stamp-extra-info', self.tasks, metadata)
         self.packages_dynamic = self.listvar('PACKAGES_DYNAMIC', metadata)
         self.depends          = self.depvar('DEPENDS', metadata)
@@ -151,6 +152,7 @@ class CoreRecipeInfo(RecipeInfoCommon):
         cachedata.pkg_dp = {}
 
         cachedata.stamp = {}
+        cachedata.stamp_base = {}
         cachedata.stamp_extrainfo = {}
         cachedata.fn_provides = {}
         cachedata.pn_provides = defaultdict(list)
@@ -183,6 +185,7 @@ class CoreRecipeInfo(RecipeInfoCommon):
         cachedata.pkg_pepvpr[fn] = (self.pe, self.pv, self.pr)
         cachedata.pkg_dp[fn] = self.defaultpref
         cachedata.stamp[fn] = self.stamp
+        cachedata.stamp_base[fn] = self.stamp_base
         cachedata.stamp_extrainfo[fn] = self.stamp_extrainfo
 
         provides = [self.pn]