refactor(lib/attestation): build option for plat token buffer size

Until now, the size of the buffer for the platform token received
from EL3 was fixed at 4kB. Now it can be configured through build
option ATTEST_PLAT_TOKEN_SIZE (defaults to 4kB).

Also suppresses a coverity misra error about uninitialized
`platform_token_len` variable.

Change-Id: Ib11b325dbc1db930e517ce55c2f61dc36f594b8b
Signed-off-by: Juan Pablo Conde <juanpablo.conde@arm.com>