This commit makes all our `.mk` files compliant with the Buildroot
coding style:
https://buildroot.uclibc.org/downloads/manual/manual.html#_coding_style

Specifically, this commit addresses 571 style offenses for the following
rules:

* consecutive empty lines
* empty line at end of file
* expected indent with tabs
* line contains trailing whitespace
* missing newline at end of file
* remove default value of _SOURCE variable
* remove trailing backslash
* should be 1 hash
* should be 80 hashes
* should be a blank line
* unexpected indent with tabs
* useless default value
* possible typo: ARMSNES_SITE
* possible typo: ARMSNES_TARGET
* possible typo: ARMSNES_VERSION
* possible typo: GLUPEN64_PRE_CONFIGURE_HOOKS
* possible typo: INITRAMFS_DIR
* possible typo: KODI_PLUGIN_TARGET_DIR
* possible typo: LIBRETRO_BOARD
* possible typo: LIBRETRO_PLATFORM
* possible typo: MUPEN64PLUS_GL_CFLAGS
* possible typo: PKGCONFIG_CONFIG
* possible typo: SDL_CONFIG
* possible typo: SDL_FLAGS
* possible typo: SDL_LIBS

Buildroot check script has been run from the project root directory as
follows:

```shell
buildroot/utils/check-package {,custom/}package/**/*.mk
```