build: Rename .service files to service.in

Rename the .service files to .service.in to allow service files
to support config variable substitution. Ex, this line in .in:
ExecStart=

build: Rename .service files to service.in

Rename the .service files to .service.in to allow service files
to support config variable substitution. Ex, this line in .in:
ExecStart=/usr/bin/obmc-flash-bmc ubiro @IMG_UPLOAD_DIR@
would become this in .service file:
ExecStart=/usr/bin/obmc-flash-bmc ubiro "/tmp/images"

This allows the service files to use the user defined config
values instead of making assumptions that the default is used.

This commit only renames the service files. The subsequent
commit will make use of this feature.

In addition, the subdir meson.build files were deleted with
this commit. The reason is that the service files need to be
moved to the main meson.build to be able to read the conf file,
and without the service files, the subdir meson.build files are
just a couple lines long so there's not really a reason to have a
separate meson.build file.

Tested: Verified the service files were installed as before.

Change-Id: I29190d1f0664352fbee8a0ada9de959fd0269d6e
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>

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