Refactor os and arch conditionals in Makefile

[afbjorklund]

Prepare for adding another operating system for guestagents.

There was a mix between hardcoded OS and dynamic Arch.

Correct some typos in the helper functions and documentation.

Just simple things like "aget" instead of agent, nothing much.

[afbjorklund]

Only the default "guestagents" has been changed, the rest still build Linux (or nothing).

@@ -81,6 +81,7 @@ help-targets:
        @echo
        @echo  'Targets for files in _output/share/lima/:'
        @echo  '- guestagents               : Build guestagents for archs enabled by CONFIG_GUESTAGENT_ARCH_*'
+       @echo  '                              Build guestagents for oses enabled by CONFIG_GUESTAGENT_OS_*'
        @echo  '- native-guestagent         : Build guestagent for native arch'
        @echo  '- additional-guestagents    : Build guestagents for archs other than native arch'
        @echo  '- <arch>-guestagent         : Build guestagent for <arch>: $(sort $(LINUX_GUESTAGENT_ARCHS))'

[AkihiroSuda]

Thanks