SoftwareSpiMaster::acquire() is broken

[rleh]

Bugfix in modm:

[...]::acquire(void *ctx, ConfigurationHandler handler)
{
-	if (ctx == nullptr)
+	if (context == nullptr)

Code in xpcc:

xpcc/src/xpcc/architecture/platform/driver/spi/generic/spi_master_impl.hpp

Lines 68 to 70 in 871d2c0

	xpcc::SoftwareSpiMaster<SCK, MOSI, MISO>::acquire(void *ctx, ConfigurationHandler handler)
	{
	if (ctx == nullptr)

This bug is fixed in modm-io/modm#16

@salkinium Do we want to this fix in xpcc or does that complicate your xpcc-to-modm forward merging after every release?

[salkinium]

Yes, if it's simple things that do not

• move files around, or
• modify the template language inside platform/,

then it's fine. You can even have duplicate git commits, since I merge it with via content diff patch and not directly.