uClibc: drop BR2_PROGRAM_INVOCATION option

Remove the BR option and enable the configuration setting in the
uClibc defconfigs.

The BR2_PROGRAM_INVOCATION option only adds very little overhead to
uClibc, and we have a number of packages needing it, so simply always
enable it - Simplifying the kconfig logic and the number of choices
users have to make.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>