This change improves build consistency across external projects integrated
through CMake by ensuring that compiler flags defined in configuration files
are passed correctly to the toolchain. It covers the majority of use cases,
as external projects are typically also CMake-based. For projects that use
a custom build system, users will still need to specify the required flags
manually.
esp2020r1 toolchain includes a nano.specs file, which instructs GCC to
substitute libc.a with libc_nano.a.
In the build system, this simplifies handling of the nano formatting
option, eliminating LIBC and LIBM global variables.
