I'd like to manually start the ROM bootloader on a low PORTB.4 during reset. I used the following code according to instructions in the 
datasheet  how to set the non-volatile flash register BOOTCFG. But with no success so far:
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UARTprintf("BOOTCFG register is reading %8x\n",HWREG(FLASH_BOOTCFG));

 if (( HWREG(FLASH_BOOTCFG) & FLASH_BOOTCFG_NW ) != 0 )
 {
 UARTprintf("BOOTCFG has not been committed yet\n");
    
 // Write the desired boot configuration to the FMD register
 HWREG(FLASH_FMD) = (FLASH_BOOTCFG_PORT_B | FLASH_BOOTCFG_PIN_4 | FLASH_BOOTCFG_DBG1);
 UARTprintf("FMD set to 0x%8x\n", HWREG(FLASH_FMD));
    
 // Write the address of the register which we wish to commit
 HWREG(FLASH_FMA) = 0x75100000;
 UARTprintf("Register to be committed written in FMA: 0x%8x\n",HWREG(FLASH_FMA));
    
 // Write to non-volatile register (KEY + COMT bit)
 HWREG(FLASH_FMC) = (FLASH_FMC_WRKEY | FLASH_FMC_COMT);
 UARTprintf("Start committing... FMC: 0x%8x\n",HWREG(FLASH_FMC));

 //Poll COMT bit in the FMC register until the commit operation is complete
 while((HWREG(FLASH_FMC) & FLASH_FMC_COMT))
 {
 UARTprintf("Committing... FMC: 0x%8x\n",HWREG(FLASH_FMC));
 }

 UARTprintf("Committed! FMC: 0x%8x Change takes effect after reset\n",HWREG(FLASH_FMC));

 }
 else
 {
 UARTprintf("BOOTCFG has already been committed\n");
 }
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The output on the UART always reads:
Programming BOOTCFG Register
BOOTCFG register is reading fffffffe
BOOTCFG has not been committed yet
FMD set to 0x00003002
Register to be committed written in FMA: 0x75100000
Start committing... FMC: 0x 8
Committed! FMC: 0x 0 Change takes effect after reset
and this everytime I push the reset button.
I've also changed the code to write the USERREG0 instead of the BOOTCFG what worked correctly. That's why I guess implementation is actually correct.
There is an errata on this issue suggesting a special mass erease with the "unlock function" but this didn't help either.
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SOLUTION:
Just before posting this the solution finally hit me. Instead of a simple reset as stated in the datasheet (p. 315) a power cycle is needed for the changes in BOOTCFG
to take effect.
Hope this helps anybody.