Clarify what __TRUSTY__ and OPENSSL_NANOLIBC are

BoringSSL cannot support any build configurations where a platform
claims to be something but is not. Different embedded targets need their
own define.

Bug: b:291102972
Change-Id: I91db6ddd823b0d8b7b52580f7a6c498e55da6b79
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/61605
Reviewed-by: Bob Beck <bbe@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

include/openssl/target.h

diff --git a/include/openssl/target.h b/include/openssl/target.h
index 4e12652..92eaf75 100644
--- a/include/openssl/target.h
+++ b/include/openssl/target.h
@@ -81,6 +81,11 @@
 #define OPENSSL_FUCHSIA
 #endif
 
+// Trusty is Android's TEE target. See
+// https://source.android.com/docs/security/features/trusty
+//
+// Defining this on any other platform is not supported. Other embedded
+// platforms must introduce their own defines.
 #if defined(__TRUSTY__)
 #define OPENSSL_TRUSTY
 #define OPENSSL_NO_POSIX_IO
@@ -88,6 +93,9 @@
 #define OPENSSL_NO_THREADS_CORRUPT_MEMORY_AND_LEAK_SECRETS_IF_THREADED
 #endif
 
+// nanolibc is a particular minimal libc implementation. Defining this on any
+// other platform is not supported. Other embedded platforms must introduce
+// their own defines.
 #if defined(OPENSSL_NANOLIBC)
 #define OPENSSL_NO_POSIX_IO
 #define OPENSSL_NO_SOCK