Attribute 'tmpfs_percentage' is set only when build-pkgs is executed
with argument '--tmpfs_percentage'. This change verify if attribute
'tmpfs_percentage' is set before passing it to clonechroot function.
Test plan:
PASS - Run build-pkgs without --tmpfs_percentage
PASS - Run build-pkgs with --tmpfs_percentage
Closes-bug: 2083023
Change-Id: I064328d29f0c3a2592589d0e0ac964f76bb3e5ed
Signed-off-by: Dostoievski Batista <dostoievski.albinobatista@windriver.com>