Fix PROGRAM_INVOCATION handling with external toolchains

Fix PROGRAM_INVOCATION handling with external toolchains

BR2_UCLIBC_PROGRAM_INVOCATION is a toolchain configuration option,
like BR2_INET_IPV6, BR2_INET_RPC, on which some packages
depend. Therefore, it should be handled like BR2_INET_IPV6 and
BR2_INET_RPC in order to work properly with external toolchains.

Since we move it out of toolchain/uClibc/Config.in into
toolchain/Config.in.2, we rename the option to BR2_PROGRAM_INVOCATION
(since BR2_INET_RPC and others don't have UCLIBC in their name).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>