package.bbclass: moving field data process before variable process in process_pkgconfig

package.bbclass: moving field data process before variable process in process_pkgconfig

Currently, the latest version abseil-cpp contains a new library named "absl_log_internal_format", it's
basic package config(.pc file) as below:

prefix=/usr
exec_prefix=${prefix}

......

Requires: absl_config = 20230125, absl_core_headers = 20230125, absl_log_internal_append_truncated = 20230125,
absl_log_internal_config = 20230125, absl_log_internal_globals = 20230125, absl_log_severity = 20230125,
absl_strings = 20230125, absl_str_format = 20230125, absl_time = 20230125, absl_span = 20230125
......

Normally, the process_pkgconfig() would process variable data before field data in a .pc file, but in the
absl_log_internal_format, the field data in "Requires" section contains "xxxx = xxxx" format, the
process_pkgconfig() treats them as normal variable and using the setVar() in bitbake's data_smart.py
try to process. The absl_log_internal_format field data contains "_append_", this hit the setVar() checking
and finally bitbake stop building and reporting an error as below:

"Variable xxx contains an operation using the old override syntax. Please convert this layer/metadata before attempting to use with a newer bitbake."

This patch move the field data process before variable process to avoid the process_pkgconfig() treat the field
data as variable.

Signed-off-by: Xiangyu Chen <xiangyu.chen@windriver.com>
(cherry picked from commit a73e269d3e591a10bb397b94b82e3fb960112d33)
Signed-off-by: Clément Péron <peron.clem@gmail.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>