deduplicate the contents .cargo/config

using `cfg`s. Also add all the possible compilation targets to .cargo/config.

.cargo/config

@@ -1,4 +1,4 @@
-[target.thumbv6m-none-eabi]
+[target.'cfg(all(target_arch = "arm", target_os = "none"))']
 runner = 'arm-none-eabi-gdb'
 rustflags = [
   # LLD (shipped with the Rust toolchain) is used as the default linker
@@ -16,41 +16,9 @@ rustflags = [
   # "-C", "link-arg=-nostartfiles",
 ]
 
-[target.thumbv7m-none-eabi]
-runner = 'arm-none-eabi-gdb'
-rustflags = [
-  # the comments under `[target.thumbv6m-none-eabi]` also apply here
-  "-C", "link-arg=-Tlink.x",
-
-  # "-C", "linker=arm-none-eabi-ld",
-
-  # "-C", "linker=arm-none-eabi-gcc",
-  # "-C", "link-arg=-Wl,-Tlink.x",
-  # "-C", "link-arg=-nostartfiles",
-]
-
-[target.thumbv7em-none-eabi]
-runner = 'arm-none-eabi-gdb'
-rustflags = [
-  # the comments under `[target.thumbv6m-none-eabi]` also apply here
-  "-C", "link-arg=-Tlink.x",
-
-  # "-C", "linker=arm-none-eabi-ld",
-
-  # "-C", "linker=arm-none-eabi-gcc",
-  # "-C", "link-arg=-Wl,-Tlink.x",
-  # "-C", "link-arg=-nostartfiles",
-]
-
-[target.thumbv7em-none-eabihf]
-runner = 'arm-none-eabi-gdb'
-rustflags = [
-  # the comments under `[target.thumbv6m-none-eabi]` also apply here
-  "-C", "link-arg=-Tlink.x",
-
-  # "-C", "linker=arm-none-eabi-ld",
-
-  # "-C", "linker=arm-none-eabi-gcc",
-  # "-C", "link-arg=-Wl,-Tlink.x",
-  # "-C", "link-arg=-nostartfiles",
-]
+[build]
+# Pick one of these compilation targets
+# target = "thumbv6m-none-eabi"
+# target = "thumbv7m-none-eabi"
+# target = "thumbv7em-none-eabi"
+# target = "thumbv7em-none-eabihf"