gpowerd: Use MACHINE_CONFIG_TEXTPROTO for config install

Allow recipes or machine configs to override the textproto configuration
file name installed to /etc/gpowerd/gpowerd_config.textproto via
MACHINE_CONFIG_TEXTPROTO. Defaults to ${MACHINE}.textproto.

Tested: Built gpowerd for various platforms and verified config installation.
Google-Bug-Id: 544946889
Fusion-Link: fusion2, n/a
Change-Id: Ib89f349a7b191df68d7775db21afdc814fb81567
Signed-off-by: Yolkee Huang <cweihuang@google.com>