Build failed without standard library

[gaojiaqi7]

Hi,

I am trying to use mbedtls in no_std environment with x86_64-unknown-none target without standard library built-in, but I met the following errors when compiling mbedtls:

error[E0277]: the trait bound `rust_alloc::boxed::Box<SavedRawCipher>: Deserialize<'_>` is not satisfied
    --> mbedtls/src/cipher/raw/serde.rs:52:9
     |
52   |         inner_cipher_ctx: Box<SavedRawCipher>
     |         ^^^^^^^^^^^^^^^^ the trait `Deserialize<'_>` is not implemented for `rust_alloc::boxed::Box<SavedRawCipher>`
     |
     = help: the following other types implement trait `Deserialize<'de>`:
               &'a [u8]
               &'a str
               ()
               (T0, T1)
               (T0, T1, T2)
               (T0, T1, T2, T3)
               (T0, T1, T2, T3, T4)
               (T0, T1, T2, T3, T4, T5)
             and 102 others
note: required by a bound in `next_value`
    --> /home/tdvf/.cargo/registry/src/github.com-1ecc6299db9ec823/serde-1.0.101/src/de/mod.rs:1845:12
     |
1845 |         V: Deserialize<'de>,
     |            ^^^^^^^^^^^^^^^^ required by this bound in `next_value`

Some errors have detailed explanations: E0277, E0407.
For more information about an error, try `rustc --explain E0277`.
error: could not compile `mbedtls` due to 4 previous errors

error[E0405]: cannot find trait `Default` in this scope
    --> /home/tdvf/.cargo/registry/src/github.com-1ecc6299db9ec823/byteorder-1.3.2/src/lib.rs:1831:6
     |
1831 | impl Default for LittleEndian {
     |      ^^^^^^^ not found in this scope
     |
help: consider importing this trait
     |
83   | use core::default::Default;
     |

Some errors have detailed explanations: E0405, E0425, E0463.
For more information about an error, try `rustc --explain E0405`.
error: could not compile `byteorder` due to 173 previous errors

The error message is too long, so I only pasted part of it here.
The build command I'm using is:

cargo build -p mbedtls -Zbuild-std=core,alloc,compiler_builtins --target=x86_64-unknown-none --no-default-features --features="no_std_deps,rdrand"

The same result applies to:

cargo build -p mbedtls -Zbuild-std=core,alloc,compiler_builtins --target=x86_64-unknown-linux-gnu--no-default-features --features="no_std_deps,rdrand"

[gaojiaqi7]

From the compiler output, it seems the default features of byteorder should not be enabled and the alloc feature of serde should be enabled under no_std.
https://github.com/BurntSushi/byteorder/blob/master/Cargo.toml#L25

I submitted a pull request #209 trying to fix this error. Please take a review, thanks🙂