Convert all make rules to be overridable, clean up typos

[andrewimeson]

Various guides on the internet suggest editing rules/config to enable features. This slightly complicates maintaining a build, because you either have to git stash (and hope that when you re-apply a change to the file hasn't caused a conflict) before pulling, or commit your changes and merge or rebase upstream changes. Prior to this these were the only configurable values in the file:

• SONIC_DPKG_CACHE_METHOD
• SONIC_DPKG_CACHE_SOURCE
• INCLUDE_KUBERNETES_MASTER
• SONIC_ENABLE_IMAGE_SIGNATURE
• SONIC_ENABLE_SECUREBOOT_SIGNATURE
• PACKAGE_URL_PREFIX
• SONIC_VERSION_CONTROL_COMPONENTS
• REGISTRY_PORT
• REGISTRY_SERVER
• ENABLE_ASAN
• DEFAULT_CONTAINER_REGISTRY
• ENABLE_FIPS_FEATURE
• ENABLE_FIPS
• SONIC_SLAVE_DOCKER_DRIVER

Why I did it

Since these are build flags, it shouldn't require modifying source-controlled files to build.

How I did it

Switching from variable assignment to optional variable assignment if unset.

How to verify it

$ make init
$ make configure PLATFORM=broadcom
$ make ENABLE_SFLOW_DROPMON=y SONIC_BUILD_JOBS=4 target/sonic-broadcom.bin
…
SONiC Build System

Build Configuration
"CONFIGURED_PLATFORM"             : "broadcom"
SONiC Build System

Build Configuration

…
"ENABLE_SFLOW_DROPMON"            : "y"

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205

Description for the changelog

Allow overriding rules/config values with make

A picture of a cute animal (not mandatory but encouraged)

Bartalina the house sheep @ 1-2 days old

[andrewimeson]

I learned on the mailing list that I can create rules/config.user which will get read in after rules/config.

I think it may make sense to support both? It's a little confusing to have only some flags configureable via make FLAG_NAME=y ….

[andrewimeson]

example of an issue due to this confusion
#12362

[saiarcot895]

Contrary to the original description, this might actually be disabled by default. It's also not used anywhere (besides defining some log_* color functions), as far as I can tell, but it's probably best to default this to n instead.

[saiarcot895]

example of an issue due to this confusion #12362

This is actually due to a different reason, I think related to Makefile.work not propagating this variable into the slave docker container. However, I think it turns out that the intended value for that variable does get applied due to it getting stored in src/sonic-build-hooks/buildinfo/config/buildinfo.config. Regardless, this should probably get fixed to at least show the correct value.