Rename the generic name `ctx` of `mbedtls_platform_context`
in `platform_alt.c` to a specific name `plat_ctx`, to avoid conflicts
when used as external in crypto modules.
The DEVICE_FOO macros are always defined (either 0 or 1).
This patch replaces any instances of a define check on a DEVICE_FOO
macro with value test instead.
Signed-off-by: Alastair D'Silva <alastair@d-silva.org>