1. Added support for LSDK-20.12
2. Added initial support for secure boot; there are few atf patches that
will be added later
3. Fixed documentation in README.md withregards SPI flashing to use
cmp.b instead of cmp (the size is given in bytes)
4. Added ATF debug/release build environment variable
Signed-off-by: Rabeeh Khoury <rabeeh@solid-run.com>