Add .DS_Store to .gitignore

Instruct git to ignore these OS X files.
Cast to matching enumeration type instead of uint32_t

These were done to silence GCC warnings and fix potential bugs where
they would never be equal when the enumeration wasn't a 32-bit type.

For example, common/pinmap_common.c used to contain this code:
    if (pin == (uint32_t)NC)
I switched it to:
    if (pin == (PinName)NC)

I wonder why this casting to uint32_t was done in the first place?
Maybe another supported compiler requires it?
Fix operator precedence warning in can_api.c

The original code was:
    if(LPC_CAN1->IER | LPC_CAN2->IER != 0) {

This would actually be interpreted as:
    if(LPC_CAN1->IER | (LPC_CAN2->IER != 0)) {
I simplified it to:
    if(LPC_CAN1->IER | LPC_CAN2->IER) {
With the comparison removed, the GCC warning no longer fires since the
user's intent is no longer unclear.  However, the end result should be
the same.
Silence signed/unsigned comparison warnings in GCC.

Why do the wait APIs take a signed integer if they are going to be
compared to unsigned quantities?
Cast to matching enumeration type instead of uint32_t

This commit targets the LPC11U24 code, whereas a previous one
targetted similar issues in the LPC1768 mbed HAL code.

These changes were made to silence GCC warnings and fix potential bugs
where they would never be equal when the enumeration wasn't a 32-bit
type.

For example, pinmap.c used to contain this code:
    if (pin == (uint32_t)NC) return;
I switched it to:
    if (pin == (PinName)NC) return;

I wonder why this casting to uint32_t was done in the first place?
Maybe another supported compiler requires it?
Cast to matching enumeration type instead of uint32_t

This commit targets the KL25Z code, whereas previous ones targetted
similar issues in the LPC1768 and LPC11U24 mbed HAL code.

These changes were made to silence GCC warnings and fix potential bugs
where they would never be equal when the enumeration wasn't a 32-bit
type.

For example, pinmap.c used to contain this code:
    if (pin == (uint32_t)NC) return;
I switched it to:
    if (pin == (PinName)NC) return;

I wonder why this casting to uint32_t was done in the first place?
Maybe another supported compiler requires it?
Silence signed/unsigned comparison warnings in GCC.

i2c_frequency() compares a uint32_t ref variable to the int hz
function parameter passed in by the caller.  I forced this to be an
uint32_t comparison.

i2c_slave_write() declared i and count variables to be of type uint32_t
but used them as int type throughout the code (in comparisons and
returns) so I switched them to be of signed int type.

spi_frequency() contains a change similar to that made in
i2c_frequency().
Merge pull request #24 from adamgreen/gccWarnFixMbedSDK

Update mbed SDK sources to silence GCC warnings