community/binutils-{avr,cross-embedded}: Merge packages

This MR does two things:

• Add AVR support to community/binutils-cross-embedded via subpackage binutils-avr
  • Also added current maintainer of binutils-avr as co-maintainer to binutils-cross-embedded Stefan Wagner seems to not have contributed to binutils-avr since 2016 and is unresponsive to emails.
• Drop community/binutils-avr as standalone package

While the AVR toolchain does have a unique C library (avr-libc instead of the commonly used newlib), the binutils package does not differ in any regard to the other embedded toolchains. Thus, no need to maintain two distinct packages for binutils.