bsp/mpc5200: Add MPC5200_BOARD prefix
bsp/mpc5200: Fix cache handling
bsp/mpc5200: Add and use enable_bat_4_to_7()
bsp/mpc5200: Do not initialize debug registers
bsp/mpc5200: Set XLB timeout values
bsp/mpc5200: Remove Erratum 342/339 comment
bsp/mpc5200: Change ADREN register defines
bsp/mpc5200: Move comment
bsp/mpc5200: Move CSBOOTROM_VAL definition
bsp/mpc5200: Set SDELAY register
bsp/mpc5200: Change SDRAM initialization

Change SDRAM initialization according to application note AN3221.
bsp/mpc5200: New BSP variant BRS6L
sapi: Fix warnings
mips: Fix warnings
libtests/exit01: New test
libtests/exit02: New test
README: Rewrite and reduce

Delete old bit-rotting README files and rewrite the README to point
readers toward authoritative sources of documentation.
README: Update README and INSTALL to try to be GNU'ish.