boot: bootutil: Fix encryption/decryption size during copy

During region copy, when chunk contains both header and TLVs,
the encryption/decryption size is too large, as it takes into account
TLVs only.
The fix modifies the encryption/decryption size to take into
account both header and TLVs.

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