Add support for creating ext4 images with mke2fs

We are investigating replacing make_ext4fs with the upstream tool mke2fs.
To mitigate the trouble that may arise if the new tool behave differently
compared to the old one, there will be a transition period.

Devices that want to use the new way of creating ext4 images can set the
variable "TARGET_USES_MKE2FS" to true in their BoardConfig.mk
By default, the build system will choose the old tool 'make_ext4fs'.

Test: m otapackage with TARGET_USES_MKE2FS={,false,true}
Change-Id: I282bcb9efe335a86c53986283090ca947d65c7f8