1.Add sign to indicate negative and positive
2.Fix case where there are negative numbers in the operation
3.expand to 512 bytes to support rsa2048
Signed-off-by: makejian <makejian@xiaomi.com>
porting from https://github.com/kokke/tiny-bignum-c commit ac136565378c624365e0f5f556d386b3966bff32 and adapting to the nuttx
Signed-off-by: makejian <makejian@xiaomi.com>