RISC-V: fixes

[kriskwiatkowski]

Two small commits to enable RISC-V builds

• disco_generateKeyPair can't be declared inline as compiler may decide not to inline it (see C99 6.7.4 for complicated explanation ). I was getting following errors when compiling for RISC-V/32

  /usr/local/lib/gcc/riscv64-unknown-elf/8.2.0/../../../../riscv64-unknown-elf/bin/ld: bin/tests/test_disco.o: in function `test_N':
  test_disco.c:(.text+0x2e): undefined reference to `disco_generateKeyPair'
  /usr/local/lib/gcc/riscv64-unknown-elf/8.2.0/../../../../riscv64-unknown-elf/bin/ld: test_disco.c:(.text+0x20c): undefined reference to `disco_generateKeyPair'
  /usr/local/lib/gcc/riscv64-unknown-elf/8.2.0/../../../../riscv64-unknown-elf/bin/ld: bin/tests/test_disco.o: in function `.L0 ':
  test_disco.c:(.text+0x554): undefined reference to `disco_generateKeyPair'
  /usr/local/lib/gcc/riscv64-unknown-elf/8.2.0/../../../../riscv64-unknown-elf/bin/ld: test_disco.c:(.text+0x56c): undefined reference to `disco_generateKeyPair'
  /usr/local/lib/gcc/riscv64-unknown-elf/8.2.0/../../../../riscv64-unknown-elf/bin/ld: bin/src/disco_asymmetric.o: in function `.L97':
  disco_asymmetric.c:(.text+0x53a): undefined reference to `disco_generateKeyPair'

• As signed values are compared to unsigned values compiler may produce warnings, which fails a build if compiling with -Wextra -pedantic

[kriskwiatkowski]

Other than that, this library currently compiles for RISC-V but seems there are 2 problems:

• test_N aborts - needs investigation
• more importantly I needed to disable random number generation because I'm not sure how to get a random number from my microcontroller (also needs investigation).

I wonder if it makes sense to add DRBG as small devices may have crappy/slow PRNG. Hardware PRNG would be used only to initialize DRBG (and reseeding) which maybe could improve random generation. This would obviously increase code size, so must be opt-in/opt-out option selected at compile time

RISC-V ongoing work here: https://github.com/henrydcase/riscv-sandbox/tree/disco/disco

[mimoo]

Hey @henrydcase

Awesome thanks for the PR!

for DRBG you have this: https://github.com/mimoo/disco-c/blob/master/lib/disco_symmetric.h#L53