Rename the unhelpful '.platform' file to 'platform'

Having the platform configuration file be a hidden file is one of the
most tragic and newbie-unfriendly decisions ever made by TinyOS. It
contains a fair bit of magic that is remarkably hard to find if you
aren't familiar with TinyOS. Worse yet, the error message if it is
missing used to give no indication whatsoever about what was wrong or
that a mystery hidden file should even exist. The old error message
produced after temporarily renaming epic's .platform file was:

    mkdir -p build/epic
        compiling BlinkAppC to a epic binary
    ncc -o build/epic/main.exe  -Os -I/home/ppannuto/code/shed/tinyos/tos/platforms/epic/chips/cc2520 -fnesc-separator=__ -Wall -Wshadow -Wnesc-all -target=epic -fnesc-cfile=build/epic/app.c -board= -DDEFINED_TOS_AM_GROUP=0x22 -DIDENT_APPNAME=\"BlinkAppC\" -DIDENT_USERNAME=\"ppannuto\" -DIDENT_HOSTNAME=\"pfet-vm-fedora\" -DIDENT_USERHASH=0x86e1c0ccL -DIDENT_TIMESTAMP=0x517ef83bL -DIDENT_UIDHASH=0xc3c958e6L  BlinkAppC.nc -lm
    Unknown target epic
    Known targets for TinyOS directory /home/ppannuto/code/tinyos-main/tos
    and the specified include directories are:
    mulle btnode3 eyesIFXv1 eyesIFXv2 mica2dot intelmote2 null shimmer2 tinynode shimmer2r span iris mica2 sam3u_ek ucmini telosa micaz shimmer telosb sam3s_ek z1
    make: *** [exe0] Error 2

The new error message helpfully informs the user that the needed
'platform' file is missing and also points to other directories that
have examples of proper platform files:

    Missing platform file for target epic
    For the TinyOS directory /home/ppannuto/code/tinyos-main/tos
    the following targets have valid platform files:
      btnode3 eyesIFXv1 eyesIFXv2 intelmote2 iris mica2 mica2dot micaz mulle null sam3s_ek sam3u_ek shimmer shimmer2 shimmer2r span telosa telosb tinynode ucmini z1
Apparently .sensor is a thing too
Also should to pick up .default-platform

Though it doesn't look like anyone's using it besides the root