[Title] Modify usage of kdbgs for environment variables and syntax of getenv to print all environment variables
[Title] Modify usage of kdbgs for environment variables and syntax of getenv to print all environment variables

[Description] modify usages of setenv, getenv and unsetenv and a syntax of getenv to display all environment variables
[Module] utils/kdbg
[Board] all
[Reference] none
[Author] ahreum.jeong

Change-Id: I9e063e8fcbde40fcf122d2e970dcd682296643f3
Signed-off-by: ahreum.jeong <33863a1723a2843378ef12cc55c193a40f777104@samsung.com>

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