Rework "Building UEFI programs" sections in the readmes

These sections were mostly duplicated between the top-level readme and the
`uefi` package's readme. In the top-level readme, it had a broken link to the
`BUILDING.md` document which has been deleted.

Since we now have "Documentation" sections in both those readmes, the "Building
UEFI programs" section was mostly redundent, except for the part about MSRV. Add
new MSRV sections, and in the top-level readme just link to the `uefi` package's
readme so that we don't have to update both places when the MSRV changes.

Author: nicholasbishop

README.md

@@ -89,14 +89,9 @@ most of the library's functionality.
 Check out the testing project's [`README.md`](uefi-test-runner/README.md) for
 prerequisites for running the tests.
 
-## Building UEFI programs
+## MSRV
 
-For instructions on how to create your own UEFI apps, see the [BUILDING.md](BUILDING.md) file.
-
-The uefi-rs crates currently require some [unstable features].
-The nightly MSRV is currently 2022-11-22.
-
-[unstable features]: https://github.com/rust-osdev/uefi-rs/issues/452
+See the [uefi package's README](uefi/README.md#MSRV).
 
 ## Contributing
 

uefi/README.md

@@ -47,15 +47,12 @@ For additional information, refer to the [UEFI specification][spec].
 [spec]: http://www.uefi.org/specifications
 [uefi-rs book]: https://rust-osdev.github.io/uefi-rs/HEAD
 
-## Building UEFI programs
-
-For instructions on how to create your own UEFI apps, see the [tutorial].
+## MSRV
 
 The uefi-rs crates currently require some [unstable features].
 The nightly MSRV is currently 2022-11-22.
 
 [unstable features]: https://github.com/rust-osdev/uefi-rs/issues/452
-[tutorial]: https://rust-osdev.github.io/uefi-rs/HEAD/tutorial/introduction.html
 
 ## License