tee_acipher_rsadorep: fix handling of zero-padding in output

- Always remove leading null bytes in output buffer (previously this was done
only for PK_PRIVATE keys)
- Leave one null byte when rsa_exptmod() output is all zeroes (do not return
an empty buffer)
- Fix output buffer length check (take padding into account)
TEE_ROUNDxxx renamed in ROUNDxxx in libutee

This change is to have the same macro names in core
part and libutee part
Make more common implementation of orly2 and cannes platforms

This will prepare PLAT_FLAVOR of orly2 / cannes
plat-orly2 and plat-cannes merged in plat-stm

PLATFORM_FLAVOR is used to distinguish orly2 and cannes

Change-Id: Iaed89451f704120e29b0b0adb83627f11bf9df48