bootutil: Fix erase of trailer when located in scratch area

Trailer is temporary located in scratch when data of image goes up to last
sector of primary slot. At the end of the swap procedure, the temporary
trailer in scratch must be erased to prevent mcuboot from reading a stale
status from the scratch area. The erase tentative was not correct, this
is fixed.Issue is easily reproduced with an image size of one sector.

Signed-off-by: Stephane Le Roy <stephane.leroy@st.com>
Signed-off-by: Michel Jaouen <michel.jaouen@st.com>