boot_image modules inside APEX have correct names

This change fixed the problem that boot_image modules when installed to
an APEX get incorrect names like signed.img. The filename now is
"<modulename>.img" and can be overridden via the new `stem` property.

Bug: 178978059
Test: m
Change-Id: I1b25db04a4a2d888371b174c42f91a0cea87b877