Fastboot: use flash erase & logical block size for building userdata

If the bootloader provides erase-block-size and logical-block-size
getvar variables, then pass these to libext4_utils when building a new
userdata image. This info is used to tune stride and stripe-width.

Bug: 33243520
Test: Modify fb_getvar to return values for "erase-block-size" and
"logical-block-size" and check that fastboot -w sets userdata
parameters correctly.
Signed-off-by: Connor O'Brien <connoro@google.com>

Change-Id: Id48b7a3ebb9074983a4422a79a64dcb437c0f888