Hi Chris,
I recently went through this for a project that I'm working on. I got it working and here's what I did. (Disclaimer: it's not elegant.)
Background
I have a K70FX-based board and a Kingston KE4CN2H5C eMMC part. The production boards will have 5.0 parts but I had 4.5 and 5.0 samples. I used both in the development/debugging.
Problem
As you have correctly observed, the sdcard/esdhc driver in MQX 4.1 doesn't support eMMC parts. Not even close. Not only does it not follow the state machine in JEDEC 84-B45, it doesn't follow the Freescale SD/MMC Deep Dive from FTF2010 ( WBNR_FTF10_NET_F0598_PDF.pdf , page 17).
My Solution
After reviewing the driver code I decided that I didn't want to modify _esdhc_get_card_type(). Instead, I wrote _esdhc_get_mmc_type() and added it to esdh.c. In addition I created sdcard_emmc in Peripheral_IO_Drivers/sdcard. This way I can keep most of the eMMC code in one place. I control which driver is used (either sdcard/sdcard_esdhc or sdcard/sdcard_emmc) by a manifest constant in user_config.h near the bottom of the file:
user_config.h
/*
** include common settings
*/
#define MQX_USE_IO_OLD 1
#define BSPCFG_USE_EMMC 1
Then in BSP_Files/init_sdcard.c:
init_sdcard.c
const SDCARD_INIT_STRUCT _bsp_sdcard0_init = {
#if BSPCFG_USE_EMMC
_io_sdcard_emmc_init,
_io_sdcard_emmc_read_block,
_io_sdcard_emmc_write_block,
ESDHC_BUS_WIDTH_4BIT
#else
_io_sdcard_esdhc_init,
_io_sdcard_esdhc_read_block,
_io_sdcard_esdhc_write_block,
ESDHC_BUS_WIDTH_4BIT
#endif
};
Finally in Peripheral_IO_Drivers/esdhc.c:_esdhc_ioctl() in the code for IO_IOCTL_ESDHC_GET_CARD I did the following:
#if BSPCFG_USE_EMMC
/* Recognize inserted card */
val = _esdhc_get_mmc_type(esdhc_device_ptr);
#else
val = _esdhc_get_card_type(esdhc_device_ptr);
#endif
if (val != ESDHC_OK)
{
esdhc_device_ptr->CARD = ESDHC_CARD_UNKNOWN;
}
Final Comments
I set my eMMC devices to 4-bit mode and they work fine. I have not tested the _set_sd_high_speed_mode() function in sdcard_emmc.c. I don't need it and it's a copy from sdcard_esdhc.c. My parts are 4GB parts. That means that the sector count is stored in EXT_CSD. I wrote code to read it. If you have an eMMC part with capacity less than 2GB then I think the sector count is in the CSD. I think that should be the same as SD cards.
When setting the relative address with CMD3 I wasn't sure what to do. I knew address 0x1 was illegal, so I chose 0x2. (Remember, I am not an expert!) 
For 5.0 parts I found that I had to add a delay in my application right before _io_part_mgr_install(). I don't like that solution and I need to go back and take a look at it again (but there are too many other pressing issues.
Also, the CMD line is supposed to be in open drain mode in the beginning and later changed to push-pull. So in init_gpio.c I have the following:
#if BSPCFG_USE_EMMC
pctl->PCR[3] = value & (PORT_PCR_MUX(4) | PORT_PCR_ODE_MASK | PORT_PCR_PS_MASK | PORT_PCR_PE_MASK | PORT_PCR_DSE_MASK); /* ESDHC.CMD */
#else
pctl->PCR[3] = value & (PORT_PCR_MUX(4) | PORT_PCR_PS_MASK | PORT_PCR_PE_MASK | PORT_PCR_DSE_MASK); /* ESDHC.CMD */
#endif
And then later in _io_sdcard_emmc_init() I change it to push-pull.
Good luck!
Paul
