Only include dependencies when binary feature is enabled

This change vastly improves compile times when only `lib.rs` is compiled. However, it is a breaking change.

Author: phil-opp

Cargo.toml

@@ -9,10 +9,10 @@ edition = "2018"
 build = "build.rs"
 
 [dependencies]
-xmas-elf = "0.6.2"
-x86_64 = "0.7.2"
-usize_conversions = "0.2.0"
-fixedvec = "0.2.3"
+xmas-elf = { version = "0.6.2", optional = true }
+x86_64 = { version = "0.7.2", optional = true }
+usize_conversions = { version = "0.2.0", optional = true }
+fixedvec = { version = "0.2.3", optional = true }
 
 [dependencies.font8x8]
 version = "0.2.4"
@@ -25,6 +25,7 @@ llvm-tools = "0.1"
 
 [features]
 default = []
+binary = ["xmas-elf", "x86_64", "usize_conversions", "fixedvec"]
 vga_320x200 = ["font8x8"]
 recursive_page_table = []
 map_physical_memory = []

README.md

@@ -30,10 +30,10 @@ As an example, you can build the bootloader with example kernel from the `exampl
 cd example-kernel
 cargo xbuild
 cd ..
-KERNEL=example-kernel/target/x86_64-example-kernel/debug/example-kernel cargo xbuild --release
+KERNEL=example-kernel/target/x86_64-example-kernel/debug/example-kernel cargo xbuild --release --features binary
 ```
 
-This results in a bootloader executable at `target/x86_64-bootloader.json/release/bootloader`. This executable is still an ELF file, which can't be run directly.
+The `binary` feature is required to enable the dependencies required for compiling the bootloader executable. The command results in a bootloader executable at `target/x86_64-bootloader.json/release/bootloader`. This executable is still an ELF file, which can't be run directly.
 
 ## Run