Make check for loader features caseless

[lytboris_gmail.com]

Summary

Trying to enable GZIP loader feature by adding LOADER_GZIP_SUPPORT=YES into src.conf while building a CI infrastructure would lead to disabling this feature as current makefile code checks for yes/no values in a case-sensitive manner.

This oatch makes these checks case-insensitive and adds some hints for future us.

Test Plan

1. Put LOADER_GZIP_SUPPORT=YES into src.conf
2. Check strings loader for zf_open in symbols list.

[vexeduxr]

LOADER_DISK_SUPPORT is also checked in stand/defs.mk, otherwise looks good to me.

Interestingly enough LOADER_TFTP_SUPPORT is documented with uppercase values in multiple places.

[lytboris_gmail.com]

Updated diff, altered one check in stand/defs.mk

[imp]

all the other build system things, like for MK_FOO are lower case only. I worry this would cause confusion...

[lytboris_gmail.com]

In D58381#1339766, @imp wrote:

all the other build system things, like for MK_FOO are lower case only. I worry this would cause confusion...

We've got into trouble with unbootable servers twice becase of LOADER_GZIP_SUPPORT=YES :)
Not sure if LOADER_FEATURES_INCLUDE/LOADER_FEATURES_EXCLUDE approach with subsequent :M checks is any better.
Another options is to break the build in case someone uses wrong words (on instead of yes).

Which would you prefer?